configs/rock5b: add firmware for the RTL8125 NIC

configs/rock5b: add firmware for the RTL8125 NIC

Add the proper firmware to avoid the following warning + error during
boot:
r8169 0004:41:00.0: Direct firmware load for rtl_nic/rtl8125b-2.fw failed with error -2
r8169 0004:41:00.0: Unable to load firmware rtl_nic/rtl8125b-2.fw (-2)

Note that the NIC appears functional even without the proper firmware.
However, add the firmware anyway, since the error messages are distracting
and clutters dmesg.

Signed-off-by: Niklas Cassel <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>