ARM: UniPhier: set MACH_PH1_PRO4 as default SoC

One disadvantage of commit a26cd04920dc (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>
diff --git a/arch/arm/mach-uniphier/Kconfig b/arch/arm/mach-uniphier/Kconfig
index 73b8676..feda49e 100644
--- a/arch/arm/mach-uniphier/Kconfig
+++ b/arch/arm/mach-uniphier/Kconfig
@@ -8,7 +8,7 @@
 
 choice
 	prompt "UniPhier SoC select"
-	optional
+	default MACH_PH1_PRO4
 
 config MACH_PH1_PRO4
 	bool "PH1-Pro4"