sh: sh_bios detection.

This adds some VBR sanity checks in the sh_bios code to ensure that the
BIOS VBR is in range before blindly trapping in to it. This permits
boards with varying boot loader configurations to always leave support
for sh-bios enabled and it will just be disabled at run-time if not
found.

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