ARM: OMAP2+: hwmod: manage the wake-up latency constraints

The OMAP PM code implements a handler for the per-device PM QoS framework.
The handler queries the omap_hwmod layer in order to manage the power domains
wake-up latency constraints. Hwmod retrieves the correct power domain
and if it exists it calls the corresponding power domain function.

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: cleaned some documentation; split set/remove constraint
 functions; modified to return -EINVAL until underlying code is ready]
Signed-off-by: Paul Walmsley <paul@pwsan.com>