ipq4019: patch initialization of Lantiq DSL on FritzBox 7530.

ipq4019: patch initialization of Lantiq DSL on FritzBox 7530.

Some VRX518 modems fail to initialize properly with the error message
"dc_ep_clk_on failed". As a result, the DSL data path doesn't work.

This hack, which is based on code from the FRITZ!Box 7530 GPL archive,
fixes the issue. It changes the PCIe vendor/device ID to values matching
a Lantiq SoC. It also appears to emulate a Lantiq CPU ID register for
connected PCIe devices, by remapping the matching address area to a
specially crafted buffer using the address translation unit.

A dedicated compatible is created to activate this in
the device tree, so this shouldn't affect any devices other than
FRITZ!Box 7530/7520.

Original investigation was done in https://github.com/janh/openwrt/commit/59f52125178146a9dc44290c11c63e5e029e8044 which used the "avm,host_magic" property to enabled the patch.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Florian Maurer <f.maurer@outlook.de>
Link: https://github.com/openwrt/openwrt/pull/17622
Signed-off-by: Robert Marko <robimarko@gmail.com>