realtek: rework hardware-assisted indirect mdio access

realtek: rework hardware-assisted indirect mdio access

With commit b53202a8c3f7 ("realtek: switch to use generic MDIO accessor functions")
the phy access logic was enhanced. A quite big kernel patch was introduced that
allowed to make use of hardware-assisted page access like we have in the realtek
target. Basically it works the following way

- The enhanced bus intercepts page write accesses
- Currently selected pages are stored in internal vars.
- Finally only all-in-one-consistent bus/register accesses are issued
- It intercepted page changes and ensured that only a complete bus call

For the details see https://github.com/openwrt/openwrt/pull/16172

Switching over to newer kernels this patch is really hard to maintain. Its heavy
modifcations exist only in the realtek target. So it does not matter if we keep
it as an kernel modification or directly include it in our driver. To make it the
future brighter we drop this patch and take over its logic. Thus the kernel will
stay totally modification-free. What do we do?

1. Up to now the bus->priv structure directly pointed to ethernet->priv. Create an
explicit private structure rtl838x_bus_priv that not only holds the ethernet->priv
pointer but also space for some bus status tracking vars as well.

2. Wherever we use a reference to ethernet->priv directly replace that by an
additional indirection over the new rtl838x_bus_priv structure.

3. Up to now the phy flag PHY_HAS_REALTEK_PAGES identified that we can use the
alternative paged access from the patch. As this will be no longer available
remove it and provide read_page/write_page functions for each possible PHY.
These functions will be pretty standard as for other Realtek PHYs.

4. The existing mdio bus read/write function rely on the classic MII_ADDR_C45
flag - one interface for two access types. This mixup will be removed on the way
to kernel 6.6. In the future there will be two pairs of access functions. One for
classic access one for c45 style access. Rewrite our functions into 3 parts:

- a classic read/write function: ready for kernel 6.6
- a new c45 read/write function: ready for kernel 6.6
- a legacy read/write wrapper: for current 5.15 for the time being

When we switch to 6.6 we only need to remove the legacy wrappers and link the
new functions. Life can be so easy.

5. The classic read/write functions will incorporate the interception logic that
was originally in the patch.

6. The package convenience functions that were embedded in the patch get lost as
well. Rewrite them inside our phy driver.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
[Minor checkpatch.pl cleanups]
Signed-off-by: Sander Vanheule <sander@svanheule.net>