sunxi: specify architecture when generating SPL boot image

As mkimage -T sunxi_egon now gains support for -A parameter, specify the
architecture when generating SPL boot image for sunxi.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>