octeon: force pcs reset to fix qca833x traffic

octeon: force pcs reset to fix qca833x traffic

There is a known bug with qca833x switches where they fail to pass
traffic without first resetting the PCS. U-Boot already has this fix,
though it uses a much newer networking stack from Marvell. This commit
backports the fix for OpenWrt.

References:
- https://github.com/u-boot/u-boot/blob/master/arch/mips/mach-octeon/cvmx-helper-sgmii.c#L197-L225
- https://github.com/u-boot/u-boot/blob/master/arch/mips/mach-octeon/cvmx-helper-sgmii.c#L701-L737

Signed-off-by: Andrew LaMarche <andrewjlamarche@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18385
Signed-off-by: Robert Marko <robimarko@gmail.com>