arm_mpam: When using MPAM0_EL1 use field names with MPAM0 prefix

Reads and writes to MPAM0_EL1 previously used field names with the MPAM1
prefix. As each MPAMn_EL1 register has the same layout, for the supported
fields, this works as expected. However, to avoid confusion, update
the field names used to match the register being written to.

Also, correct some comments to specify MPAM0_EL1 where MPAM1_EL1 was
previously erroneously mentioned.

No functional change.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>