General purpose register cannot be used for MI_MATH
calculations. ALU registers must be used.
To prevent passing general purpose register into the
EncodeMath interface, enforce a ALU register type
at compile time.
Change-Id: I98aa8605cde27e7003029d33b3ef3bcfb2306878
Signed-off-by: Sebastian Sanchez <sebastian.sanchez@intel.com>