aarch64: Add support for MPAM system registers

aarch64: Add support for MPAM system registers

This patch adds support for the system registers introduced in the
Armv8-A MPAM extension.

See https://developer.arm.com/documentation/ddi0598/latest for the
Arm ARM supplement documenting this extension.

gas/ChangeLog:

	* testsuite/gas/aarch64/mpam-bad.d: New test.
	* testsuite/gas/aarch64/mpam-bad.l: Error output.
	* testsuite/gas/aarch64/mpam-bad.s: Input.
	* testsuite/gas/aarch64/mpam.d: New test.
	* testsuite/gas/aarch64/mpam.s: Input.

opcodes/ChangeLog:

	* aarch64-opc.c (aarch64_sys_regs): Add MPAM registers.