bmips: fix missing NVMEM subsystem

MAC address retrieval was switched to more generic upstream (5.13) NVMEM
based solution in commit 32adbfc ("bmips: convert mtd-mac-address
to nvmem implementation"), but NVMEM subsystem wasn't enabled in the
kernel, so fix it now.

References: openwrt#4041 (comment)
Fixes: 32adbfc ("bmips: convert mtd-mac-address to nvmem implementation")
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/bmips/config-5.10

@@ -194,6 +194,7 @@ CONFIG_NET_SWITCHDEV=y
 CONFIG_NO_EXCEPT_FILL=y
 CONFIG_NO_GENERIC_PCI_IOPORT_MAP=y
 CONFIG_NR_CPUS=2
+CONFIG_NVMEM=y
 CONFIG_OF=y
 CONFIG_OF_ADDRESS=y
 CONFIG_OF_EARLY_FLATTREE=y