sh: improve sh7785lcr power off code

Improve the sh7785lcr power off implementation to
never return. It takes some time before the board
is actually powered off, just hang after asking
the harware to power down.

This removes the serial port garbage printout.

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