ARM: dts: msm: disable PM8994 GPIO 21 on 8994 boards

This GPIO is reserved to be used by a different subsystem
and should not be available to users on the MSM.

Change-Id: I91fd06db703eb8d0ca83ca17684422c7ad981085
Signed-off-by: David Keitel <dkeitel@codeaurora.org>