ARM: OMAP2+: powerdomain/clockdomain: add a per-powerdomain spinlock

Add a per-powerdomain spinlock.  Use that instead of the clockdomain
spinlock.  Add pwrdm_lock()/pwrdm_unlock() functions to allow other
code to acquire or release the powerdomain spinlock without reaching
directly into the struct powerdomain.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Jean Pihet <jean.pihet@newoldbits.com>