KVM: arm64: Use get_raz_reg() for userspace reads of PMSWINC_EL0

PMSWINC_EL0 is a write-only register and was initially part of the VCPU
register state, but was later removed in commit 7a3ba309 ("KVM:
arm64: Remove PMSWINC_EL0 shadow register"). To prevent regressions, the
register was kept accessible from userspace as Read-As-Zero (RAZ).

The read function that is used to handle userspace reads of this
register is get_raz_id_reg(), which, while technically correct, as it
returns 0, it is not semantically correct, as PMSWINC_EL0 is not an ID
register as the function name suggests.

Add a new function, get_raz_reg(), to use it as the accessor for
PMSWINC_EL0, as to not conflate get_raz_id_reg() to handle other types
of registers.

No functional change intended.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211011105840.155815-3-alexandru.elisei@arm.com