irqchip: sunxi: Rename sunxi to sun4i

During the introduction of the Allwinner SoC platforms, sunxi was
initially meant as a generic name for all the variants of the Allwinner
SoC.

It was ok at the time of the support of only the A10 and A13 that
looks pretty much the same, but it's beginning to be troublesome with
the future addition of the Allwinner A31 (sun6i) that is quite
different, and would introduce some weird logic, where sunxi would
actually mean in some case sun4i and sun5i but without sun6i...

Moreover, it makes the compatible strings naming scheme not consistent
with other architectures, where usually for this kind of compability, we
just use the oldest SoC name that has this IP, so let's do just this.

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