ARM: uniphier: allow to enable multiple SoCs

Before this commit, the Kconfig menu in mach-uniphier only allowed us
to choose one SoC to be compiled.  Each SoC has its own defconfig file
for the build-test coverage.  Consequently, some defconfig files are
duplicated with only the difference in CONFIG_DEFAULT_DEVICE_TREE and
CONFIG_{SOC_NAME}=y.

Now, most of board-specific parameters have been moved to device trees,
so it makes sense to include init code of multiple SoCs into a single
image as long as the SoCs have similar architecture.  In fact, some
SoCs of UniPhier family are very similar:
 - PH1-LD4 and PH1-sLD8
 - PH1-LD6b and ProXstream2 (will be added in the upcoming commit)

This commit will be helpful to merge some defconfig files for better
maintainability.

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