target-arm: Implement AArch64 memory attribute registers

target-arm: Implement AArch64 memory attribute registers

Implement the AArch64 memory attribute registers. Since QEMU doesn't
model caches it does not need to care about memory attributes at all,
and we can simply make these read-as-written.

We did not previously implement the AArch32 versions of the MAIR
registers, which went unnoticed because of the overbroad TLB_LOCKDOWN
reginfo definition; provide them now to keep the 64<->32 register
relationship clear.

We already provided AMAIR registers for 32 bit as simple RAZ/WI;
extend that to provide a 64 bit RAZ/WI AMAIR_EL1.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Peter Crosthwaite <[email protected]>