arm: sunxi: Move spl_boot_device in a separate function

U-Boot itself might need to identify the boot device, for example to be
able to tell where to load the kernel from when several options are
possible.

Move the logic of spl_boot_device to a function that is compiled both for
the SPL and the main binary.

Tested-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>