net: rtl8169: add minimal support for 8125B variant

Add minimal support for 8125B version.
Changes are based on the Linux driver.
Tested on Radxa Rock 5B Rk3588 board.

Connection to a laptop worked fine in 100 Mbps mode.
1000 Mbps mode is not working at the moment.

Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>