sunxi: psci: stop modeling register layout with C structs

Since the sunxi support nowadays generally prefers #defined register
offsets instead of modeling register layouts using C structs, now is a
good time to do this for PSCI as well. This patch moves away from using
the structs `sunxi_cpucfg_reg` and `sunxi_prcm_reg` in psci.c.

The former struct and its associated header file existed only to support
PSCI code, so also delete them altogether.

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