ramips: fix missing NVMEM subsystem

ramips: fix missing NVMEM subsystem

MAC address retrieval was switched to more generic upstream (5.13) NVMEM
based solution in commit 06bb4a5018cd ("ramips: convert mtd-mac-address
to nvmem implementation") , but NVMEM subsystem wasn't enabled in the
kernel, so fix it now.

References: https://github.com/openwrt/openwrt/pull/4041#issuecomment-883322801
Fixes: 06bb4a5018cd ("ramips: convert mtd-mac-address to nvmem implementation")
Signed-off-by: David Bauer <mail@david-bauer.net>
Signed-off-by: Petr Štetiar <ynezz@true.cz> [commit message]