sunxi: prcm: Add a few registers

H6 and H616 SPL code has a few writes to unknown PRCM registers. Now
that we know what they are, let's replace magic offsets with proper
register names.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>