ARM: OMAP: omap_device: register to the per-device PM QoS framework

Implement the devices wake-up latency constraints using the global
device PM QoS notification handler which applies the constraints to the
underlying layer by calling the corresponding function at hwmod level.

Tested on OMAP3 Beagleboard and OMAP4 Pandaboard in RET/OFF using wake-up
latency constraints on MPU, CORE and PER.

Signed-off-by: Jean Pihet <j-pihet@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
[paul@pwsan.com: modified to work with omap_devices with large numbers of
 hwmods; moved code to mach-omap2/omap_device.c; added documentation; use
 notifier return codes]
Signed-off-by: Paul Walmsley <paul@pwsan.com>