arm: Centralize fixed register logic

arm: Centralize fixed register logic

When building for ARM64, we need to pass -ffixed-x18 and otherwise pass
-ffixed-r9. Rather than having this logic in two places, we can do this
once in arch/arm/config.mk. Further, while gcc will ignore being passed
both -ffixed-r9 and -ffixed-x18 and simply use -ffixed-x18, clang will
note that -ffixed-r9 is not used. Remove this duplication to also remove
the warning.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>