MIPS: Make CM GCR base configurable

Without adding a prompt for CONFIG_MIPS_CM_BASE, Kconfig doesn't allow
defconfigs to set it. Provide the prompt in order to allow for that.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>