ARM: uniphier: support USB boot mode for ProXstream2 / PH1-LD6b SoC

The USB boot code is too fat and complicated to be included in SPL
(at least for now).  So, it was implemented as a separate project
(what we call USB-loader).

The expected boot sequence is as follows:

  Boot ROM -> USB-loader -> SPL -> U-Boot proper

The USB-loader loads the SPL and U-Boot proper from a USB memory
onto the locked L2 cache.  Then, SPL needs to copy the U-Boot proper
to DRAM, so this mode looks like a NOR boot from the view of SPL.
However, we want to distinguish between (genuine) NOR boot and USB
boot in some places.

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