arm: rename default RAM region from 'SRAM' to 'RAM'

It's useful for RAMABLE_REGION to have a uniform name when
CODE_DATA_RELOCATION is supported, because otherwise the build system
needs to be aware of how the region name differs between architectures.
Since architectures tend to prefer one of 'SRAM' or 'RAM' for that
region, prefer to use 'RAM' as the more general term.

Signed-off-by: Peter Marheine <pmarheine@chromium.org>