sh: rework SuperH Mobile sleep code exception handling

This patch updates the exception handling in the sleep code
for SuperH Mobile. With the patch applied the sleep code
always rewrites the VBR and resumes from the exception vector,
re-initializes hardware and jumps straight to the original
interrupt vector.

Tested on sh7722 and sh7724 with "Sleep Mode", "Sleep Mode + SF"
and "Software Standby Mode + SF" with CONFIG_SUSPEND.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>