arm64: defconfig: Enable nvmem's rmem driver

It'll be used by the RPi4 family of boards to access its bootloader
configuration.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20210112142342.7290-1-nsaenzjulienne@suse.de