ARM: UniPhier: set MACH_PH1_PRO4 as default SoC

One disadvantage of commit a26cd049 (arch: Make board selection
choices optional) is that Kconfig could create such an insane
.config file that no board is selected.

As PH1-Pro4 is the main stream of UniPhier SoC family, rip off the
"optional" again in favor of PH1-Pro4 as the default SoC.

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