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 <cassel@kernel.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

configs/rock5b_defconfig

@@ -22,6 +22,8 @@ BR2_TARGET_UBOOT_NEEDS_ROCKCHIP_RKBIN=y
 BR2_PACKAGE_ROCKCHIP_RKBIN=y
 BR2_PACKAGE_ROCKCHIP_RKBIN_TPL_FILENAME="bin/rk35/rk3588_ddr_lp4_2112MHz_lp5_2736MHz_v1.12.bin"
 BR2_PACKAGE_ROCKCHIP_RKBIN_BL31_FILENAME="bin/rk35/rk3588_bl31_v1.40.elf"
+BR2_PACKAGE_LINUX_FIRMWARE=y
+BR2_PACKAGE_LINUX_FIRMWARE_RTL_8169=y
 
 # Kernel
 BR2_LINUX_KERNEL=y