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: Paul Mundt <lethal@linux-sh.org>