realtek: pcs: allow to configure SerDes polarity

realtek: pcs: allow to configure SerDes polarity

Allow to configure SerDes polarity in device tree. To achieve this, add
new device tree properties that can be set in the device tree definition
of the SerDes, are read by the PCS driver during probe and are applied
upon SerDes setup.

This may be required for supporting new devices as the SerDes polarity
is usually subject to the vendors board design and defined in the
hardware profile (HWP) in the SDK. Most importantly, it is quite an
important step towards being able to setup everything on our own instead
of relying on the bootloader.

Signed-off-by: Jonas Jelonek <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20648
Signed-off-by: Hauke Mehrtens <[email protected]>