ARM: sunxi: Build u-boot-sunxi-with-spl.bin on ARM64 as well

In README.sunxi64 we tell the user how to optionally create
u-boot-sunxi-with-spl.bin by manually running cat. Instead, have the
build system create the file automatically just like it does for 32-bit
sunxi boards.

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>