irqchip/sunxi-nmi: Support sun6i-a31-r-intc compatible

The R_INTC on the A31 is undocumented. It was previously supported
by the sun6i-a31-sc-nmi compatible. This compatible however required
the register region to start at the first used register, rather than
the boundaries laid out in the SoC's memory map. The new compatible
fixes the alignment, while also naming it properly.

Since the only difference between the old and new compatibles are
a fixed offset for the registers, and since the old one is deprecated,
this patch adds a set of register defines for the new compatible,
while modifying the old set to reference the new set minus a fixed
offset.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>