mtd: lpddr: fix Kconfig dependency, for I/O accessors

Not all architectures implement a writel_relaxed() accessor. Hopefully
this will change eventually, but for now, this means lpddr2_nvm.c can't
compile on some architectures.

Let's add an ARM dependency for now, and leave a comment so maybe we can
change this in the future.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Vincenzo Aliberti <vincenzo.aliberti@gmail.com>