ANDROID: mpam_arch: Hide the actual implementation of CMAX bits

Section 9.3.2 of the TRM clearly points out the implemented bits are the
most significant ones:

"""
As an example, the 3% value previously mentioned is 0x07AE. If only 8 bits
of fraction are implemented, when software stores 0x07AE into a resource
control setting, the value is shortened to the most significant bits and
stored as 0x07--.
"""

Keep the interface in accordance with the spec, but handle internally
potential differences in the actual implementation.

Change-Id: Ia10cd22b0dd17f7b99a421e8e62e996062c2c61f
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Co-developed-by: Beata Michalska <beata.michalska@arm.com>
Signed-off-by: Beata Michalska <beata.michalska@arm.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>