ARM: UniPhier: add environment sets for non-FIT Linux boot

Currently, the environment variables "norboot" and "nandboot" only
work with CONFIG_FIT, but we do not want to depend on CONFIG_FIT to
boot the kernel.

This commit adds environments useful for booting Linux with separate
uImage + ramdisk + DTB.

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