lantiq: vr200: switch to nvmem eeprom

lantiq: vr200: switch to nvmem eeprom

mediatek,mtd-eeprom predates nvmem and is being phased out.

At first glance, it looks like it's doing something useful with its
big-endian binding. However on further inspection, big-endian is handled
in the same function that mediatek,mtd-eeprom is handled. In addition,
the older mtd_read way of extracting the firmware byteswaps the data
on big endian hosts which big-endian byteswaps back. nvmem does not do
such byteswapping.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23115
Signed-off-by: Robert Marko <robimarko@gmail.com>