sunxi: spl: add support for Allwinner A523 watchdog

sunxi: spl: add support for Allwinner A523 watchdog

The watchdog in the Allwinner A523 SoC differs a bit from the one in the
previous SoCs: it lives in a separate register frame, so no longer
inside some timer device, and it manages to shuffle around some
registers a bit. But it also conveniently adds a direct reset
functionality, so we don't need to use a dummy timeout period.

Avoid introducing a new MMIO register frame C struct, but just define
the one needed register offset as a macro. Then just trigger this new
direct reset functionality in the A523 specific reset_cpu()
implementation.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>