ARM: omap2+: omap_hwmod: Set unique lock_class_key per hwmod

Add struct lock_class_key to omap_hwmod struct and use it to set unique
lockdep class per hwmod.
This will ensure that lockdep will know that each omap_hwmod->_lock should
be treated as separate class and will not give false warning about deadlock
or other issues due to nested use of hwmods.
DRA7x's ATL hwmod is one example for this since McASP can select ATL clock
as functional clock, which will trigger nested oh->_lock usage. This will
trigger false warning from lockdep validator as it is dealing with classes
and for it all hwmod clocks are the same class.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>