ARM: uniphier: simplify SoC ID get function

Currently, uniphier_get_soc_type() converts the SoC ID (this is
read from the revision register) to an enum symbol to use it for SoC
identification.  Come to think of it, there is no need for the
conversion in the first place.  Using the SoC ID from the register
as-is a straightforward way.

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