ARM: uniphier: move spl_boot_mode() to a separate file

The spl_boot_mode() is unrelated to the other code in this file.
Besides, this function is only called from common/spl/spl_mmc.c,
so it is reasonable to guard with CONFIG_SPL_MMC_SUPPORT.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>