sh: Clean up SR.RB Kconfig mess.

CPU_HAS_SR_RB is selected by both CPU_SH3 and CPU_SH4, so having a
dependency and default y on those additionally doesn't make much sense.
The select also has to be special cased for CPUs that don't support
this.

This is also something that has been abused too much as a result
of being user-visible, hence the addition of the select in the first
place. So just kill the user-visibility entirely while we're at it.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

arch/sh/Kconfig

@@ -182,9 +182,7 @@ config CPU_HAS_IPR_IRQ
 	bool
 
 config CPU_HAS_SR_RB
-	bool "CPU has SR.RB"
-	depends on CPU_SH3 || CPU_SH4
-	default y
+	bool
 	help
 	  This will enable the use of SR.RB register bank usage. Processors
 	  that are lacking this bit must have another method in place for