msm: qpnp-revid: Add api to return pmic revision id

Certain fixes in the PMIC (mainly those related to routing) do not
reflect in the revisions of individual peripherals. They show up only
in the revision id of the PMIC.

Drivers need to know whether a change is present in PMIC or not and
looking at the revision registers within the peripheral is insufficient.

So provide an api to lookup PMIC revision in the qpnp-revid driver.

Change-Id: I3bfe8a52d4eb15d1c491f0dd031e866c8b35353a
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>