arm64: rename default RAM region from 'SRAM' to 'RAM'

As with 32-bit ARM, it is useful to make the RAMABLE_REGION name uniform
across architectures so the build system does not need to be aware of
the differences when CODE_DATA_RELOCATION is supported. Generalize the
name to just 'RAM' for uniformity.

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