ARM: rmobile: Factor out SYS_SOC Kconfig option

Pull the SYS_SOC Kconfig option to avoid duplication of this option
in Kconfig.{32,64,rza1} . The default value is the same, so just set
it in one location.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/arch/arm/mach-rmobile/Kconfig b/arch/arm/mach-rmobile/Kconfig
index 9ff45f4..b69ccae 100644
--- a/arch/arm/mach-rmobile/Kconfig
+++ b/arch/arm/mach-rmobile/Kconfig
@@ -41,6 +41,9 @@
 
 endchoice
 
+config SYS_SOC
+	default "rmobile" if ARCH_RMOBILE
+
 source "arch/arm/mach-rmobile/Kconfig.32"
 source "arch/arm/mach-rmobile/Kconfig.64"
 source "arch/arm/mach-rmobile/Kconfig.rza1"