uio: Runtime PM for UIO devices

This patch modifies the uio_pdrv_genirq driver to support
Runtime PM. The power management implementation simply
runtime resumes the device at open() time and runtime
suspends it at release() time. The user space driver is
responsible for re-initializing the hardware after open().

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