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>

arch/arm/include/asm/arch-sunxi/spl.h

@@ -78,4 +78,6 @@ typedef char boot_file_head_not_multiple_of_32[1 - 2*(sizeof(struct boot_file_he
 
 #define is_boot0_magic(addr)	(memcmp((void *)addr, BOOT0_MAGIC, 8) == 0)
 
+uint32_t sunxi_get_boot_device(void);
+
 #endif