sh: move the hp6xx pm code

Move the not-so-generic pm code from arch/sh/kernel/pm.c to the
platform directory together with the rest of the hp6xx pm code.

This is done to let non-hp6xx platforms enable CONFIG_PM.

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