ARM: sunxi: Fix CPU powerdown on A83T

PRCM_PWROFF_GATING_REG has CPU0 at bit 4 on A83T. So without this
patch, instead of gating the CPU0, the whole cluster was power gated,
when shutting down first CPU in the cluster.

Fixes: 6961275e ("ARM: sun8i: smp: Add support for A83T")
Signed-off-by: Ondrej Jirman <megous@megous.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Cc: stable@vger.kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>