sh: Tidy up the optional L2 probing, wire it up for SH7786.

This tidies up the L2 probing, as it may or may not be implemented on a
CPU, regardless of whether it is supported. This converts the cvr
validity checks from BUG_ON()'s to simply clearing the CPU_HAS_L2_CACHE
flag and moving on with life.

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