arm64: allwinner: a64: add NMI (R_INTC) controller on A64

Allwinner A64 SoC features a R_INTC controller, which controls the NMI
line, and this interrupt line is usually connected to the AXP PMIC.

Add support for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
[wens@csie.org: Add fallback sun6i-a31-r-intc compatible]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>