sh: Make kgdb i-cache flushing less inept.

kgdb had its own ranged I-cache flushing routine that attempted to
duplicate the flush_icache_range() functionality, but managed to do
an explicit D-cache writeback & invalidate twice on SH-4. This is
a no-op for SH-3, and the flush_icache_range() semantics already do
what kgdb was feebly attempting to do already, so just move over to
that and kill off the wrapper.

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