uniphier: support Socionext UniPhier platform

Initial commit for Socionext UniPhier SoC support.  BL1, Bl2, and
BL31 are supported.  Refer to docs/plat/socionext-uniphier.md for
more detais.

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