ARM: uniphier: add emmcupdate command

ARM: uniphier: add emmcupdate command

The Boot ROM expects the boot image (SPL) in the Boot Partition 1.
So, updating images involves the hardware partition switch.  It might
be a bit advanced for some users.

To be user-friendly, this commit adds a useful command to update the
images; just put SPL and U-Boot proper into the public directory of
the TFTP server and execute "run emmcupdate" from the command line.

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