ARM: sunxi: Rework the restart code

The Allwinner sun6i (A31) has a slightly different watchdog, that
doesn't allow to use the already existing restart code.

Rework a bit the restart code to allow to plug in more easily different
restart handlers depending on the device tree.

In the past, we were also meaning sunxi as a generic name covering all
Allwinner SoCs. This won't be true anymore with the A31 (sun6i) that
differs pretty much from sun4i and sun5i, and we will end up having
sunxi, for sun4i and sun5i, and sun6i, which is neither consistent nor
convenient. So, while we're at it, also change sunxi to sun4i.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>