ld: Separate code and RO data sections

This prevents the execution of the read-only data.

This is done in a similar way in TF-A when the build flag
SEPARATE_CODE_AND_RODATA is enabled.

The build flag is probably not needed in TF-A Tests.

Change-Id: I2bdc0237c00377beb2febeb47207770c85036192
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>