feat(mhu): use compile flag to choose mhu version

MHUv3 and MHUv2 drivers can now be selected at build time by using
PLAT_MHU_VERSION.

Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Change-Id: I24f9e05f7969ed3be8f3261fdfed881a4ad18ba4
1 file changed