ARM: uniphier: remove part number info from the boot log

As is often the case with SoC development, slightly different
products (i.e. different part number) are developed based on the
same silicon-die.  Such fine grained information is unmaintainable.

Also, "SoC:" is a better fit that "CPU:".

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