KVM: arm64: Get rid of REG_HIDDEN_USER visibility qualifier

Now that REG_HIDDEN_USER has no direct user anymore, remove it
entirely and update all users of sysreg_hidden_user() to call
sysreg_hidden() instead.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240904082419.1982402-4-maz@kernel.org


Signed-off-by: Marc Zyngier <maz@kernel.org>