ARM: dts: msm: Enable front camera flash for SDM670 CDP/MTP

The front camera flash on SDM670 CDP/MTP is driven by LED3 and
controlled by GPIO21. Add pinctrl configuration to enable GPIO
control from flash driver.

Change-Id: I7904b337afb1d2b14c2d0d66705d399dd28dcb51
Signed-off-by: Ravi Kishore Tanuku <rktanuku@codeaurora.org>