aarch64: Allow writes to MFAR_EL3

MFAR_EL3 is a read/write register, but was incorrectly marked as
read-only
[https://developer.arm.com/documentation/ddi0601/2021-09/AArch64-Registers/MFAR-EL3--PA-Fault-Address-Register?lang=en]

opcodes/
	* aarch64-opc.c (aarch64_sys_regs): Mark mfar_el3 as read-write.

gas/
	* testsuite/gas/aarch64/rme.s: Test writing to mfar_el3.
	* testsuite/gas/aarch64/rme.d: Update accordingly.
	* testsuite/gas/aarch64/rme-invalid.s: Delete.
	* testsuite/gas/aarch64/rme-invalid.l: Likewise.
	* testsuite/gas/aarch64/rme-invalid.d: Likewise.