OMAP: HWMOD: Adding clockdomain check

This patch adds check for presence of clockdomain structure in the API
omap_hwmod_get_pwrdm before trying to access the powerdomain structure.
This will prevent unnecessary crashing of the system in case of a
clock node with out an associated clockdomain.

Signed-off-by: Thara Gopinath <thara@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>