irqchip/sunxi-nmi: Const-ify sunxi_sc_nmi_reg_offs structures

The sunxi_sc_nmi_reg_offs, which hold the register offsets for the
various variants, is never modified, and only used at init time within
the init functions referenced by IRQCHIP_DECLARE, which themselves are
tagged __init.

Const-ify the sunxi_sc_nmi_reg_offs structures, and tag them as
__initconst.

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