sh: wire up clear_user_highpage() for sh4, convert sh7705.

This wires up clear_user_highpage() on SH-4 and subsequently converts the
SH7705 32kB cache mode over to using it. Now that the SH-4 implementation
handles all of the dcache purging directly in the aliasing case, there is
no need to do this in the default clear_page() implementation.

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