Make the MHU code AML specific adding a new aml_* prefix and remove the
GXBB prefix from the register names.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Change-Id: I8f20918e29f08542bd71bd679f88e65b4efaa7d2

The MHU code is shared between all the supported platforms. Move it to
the common directory instead.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Change-Id: Iaf53122866eae85c13f772927d16836dcfa877a3