ARM: OMAP: DRA7: powerdomain: Handle missing vc/vp

DRA7 belongs to the omap4plus devices which reuse the omap4_pwrdm_operations
ops for powerdomain control. DRA7 however has no VC/VP while all the
earlier omap4plus devices did.

So use the .pwrdm_has_voltdm() ops to pass this info on to the core.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>