sh: pfc: Verify pin type encoding size at build time.

The encoding is tightly packed, and future changes (such as
pinconf-generic support) can easily lead to a situation where we violate
the encoding constraints and trample data bit/reg bits. This plugs in
some sanity checks by way of a BUILD_BUG_ON() to blow up if we fail to
fit.

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