sunxi: psci: refactor register access to separate functions

This is to prepare for R528, which does not have the typical
"CPUCFG" block; it has a "CPUX" block which provides these
same functions but is organized differently.

Moving the hardware-access bits to their own functions separates the
logic from the hardware so we can reuse the same logic.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>