ARM: uniphier: update doc/README.uniphier

 - Rephrase the toolchains section.  Leave only Linaro toolchains
   since it is the most tested these days.

 - Add build instruction for ARMv8 SoC boards

 - Add information about "ddrmphy" command

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