mtd: slram: use memremap() instead of ioremap()

Convert slram to use memremap() to map the memory it uses to back an MTD
device, as this is the proper interface for mapping memory. This change
enables normal memory to be used to back an MTD device on arm64, as arm64
prevents ioremap() being used on normal memory.

Signed-off-by: Roy Franz <roy.franz@cavium.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: David Daney <david.daney@cavium.com>
Signed-off-by: Richard Weinberger <richard@nod.at>