ARM: sunxi: Move u-boot-sunxi-with-spl.bin rule to Makefile

We're going to need this logic for 64-bit builds as well, so move it
out from under arch/arm/cpu/armv7.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>