sh: Abstracted SH-4A UBC support on hw-breakpoint core.

This is the next big chunk of hw_breakpoint support. This decouples
the SH-4A support from the core and moves it out in to its own stub,
following many of the conventions established with the perf events
layering.

In addition to extending SH-4A support to encapsulate the remainder
of the UBC channels, clock framework support for handling the UBC
interface clock is added as well, allowing for dynamic clock gating.

This also fixes up a regression introduced by the SIGTRAP handling that
broke the ksym_tracer, to the extent that the current support works well
with all of the ksym_tracer/ptrace/kgdb. The kprobes singlestep code will
follow in turn.

With this in place, the remaining UBC variants (SH-2A and SH-4) can now
be trivially plugged in.

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