Convert CONFIG_CPU_SH7751 to Kconfig

This converts the following to Kconfig:
   CONFIG_CPU_SH7751

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
index 7836869..7e6cb70 100644
--- a/arch/sh/Kconfig
+++ b/arch/sh/Kconfig
@@ -4,13 +4,17 @@
 config CPU_SH4
 	bool
 
+config CPU_SH7751
+	bool
+	select CPU_SH4
+
 choice
 	prompt "Target select"
 	optional
 
 config TARGET_R2DPLUS
 	bool "Renesas R2D-PLUS"
-	select CPU_SH4
+	select CPU_SH7751
 
 endchoice