nios2: rename default RAM region from 'SRAM' to 'RAM'

It's useful for the default RAM region to have a uniform name across
architectures so higher-level build processes do not need to have their
own awareness of the names. Since NIOS2 uniformly uses 'SRAM', change it
to 'RAM' to match most other architectures.

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