arm64: Don't try to set PSTATE for VCPUs belonging to a realm

RME doesn't allow setting the PSTATE but resets it to an architectural
value, and KVM also does not allow setting this register from user
space, so stop trying to do that.

Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>