Add MRS_EXACT_IF_DIFFERENT, MRS_HIGHER_OR_ZERO, and MRS_HIGHER.

These will be used to handle the cache CTR_EL0 register, and make some
ID register values safer.

They are:

• MRS_EXACT_IF_DIFFERENT: If the field is the same on all CPUs then use this value, otherwise use the safe value.
• MRS_HIGHER_OR_ZERO: Use the highest value, or zero if seen,
• MRS_HIGHER: Use the highest value.

Sponsored by: Arm Ltd