ARM: dts: msm: update APM compatible property for msm8996pro

Specify an MSM8996-Pro specific APM device compatible string
for MSM8996-Pro boards.  This allows software to avoid imposing
various constraints during APM switching which are not required
for MSM8996-Pro chips.

Change-Id: I0664374ca347c0f66d95422cc9bfa4efb1a151ee
Signed-off-by: David Collins <collinsd@codeaurora.org>