sh: add high impedance mode management for SIUA pins on sh7722

This improves power management for the SIUA controller on sh7722. Similar
patches might be desired for other SIU-enabled SH platforms.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>