dle path is signficiant.

In order allow the pre/post transitions to be isolated and called
per-pwrdm, change the API so passing in a specific power domain will
trigger the pre/post transtion accounting for only that specific power
domain.  Passing NULL means iterating over all power domains as is
current behavior.

Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>