xen/arm: Always access SCTLR_EL2 using READ/WRITE_SYSREG()

xen/arm: Always access SCTLR_EL2 using READ/WRITE_SYSREG()

The Armv8 specification describes the system register as a 64-bit value
on AArch64 and 32-bit value on AArch32 (same as ARMv7).

Unfortunately, Xen is accessing the system registers using
READ/WRITE_SYSREG32() which means the top 32-bit are clobbered.

This is only a latent bug so far because Xen will not yet use the top
32-bit.

There is also no change in behavior because arch/arm/arm64/head.S will
initialize SCTLR_EL2 to a sane value with the top 32-bit zeroed.

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>