ARM: dts: msm: add a VDD_APC cpr-regulator device for msm8939

Replace the regulator-apc-corner qcom,stub-regulator device node with a
qcom,cpr-regulator device node. This allows the APC power rail voltage
which is supplied by an external buck to be set based upon corners
requested from the cpr-regulator device as well as CPR hardware
recommendations.

Change-Id: Id9d9dbdb6bcfd3e41a47f85726a45c6f05425468
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>