ARM: uniphier: rework UniPhier SoC select in Kconfig

ARM: uniphier: rework UniPhier SoC select in Kconfig

The chains of "depends on <SoC_name>" in the current Kconfig is
clumsy.  The idea here is to allow users to choose a SoC group first
(SoC group consists of some SoCs that can coexist in one binary).
Then, allow to enable/disable each SoC support in the selected SoC
group.  This makes the Kconfig menu clearer.

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