sh: rework register restore code for sh3/sh4/sh4a

This patch reworks the sh3/sh4/sh4a register restore code in
the following ways:
 - break out restore_regs() from restore_all()
 - the register saving order is unchanged
 - use restore_regs() in sh_bios_handler and restore_all
 - document the function

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