[ARM] OMAP: clockdomain: add clkdm_get_pwrdm()

Add clkdm_get_pwrdm() to the clockdomain code.  It will return a
pointer to the powerdomain struct that the clockdomain is contained
within.  Used by the PM code.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>