ARM: dts: msm: add GPIO support for flash LED for MSM8937

On MSM8937, GPIO 33 is used to control rear flash LED, and
GPIO 50 is used to control front flash LED.
This change adds the support for rear flash LED only.

Change-Id: I60998d564f62bb316ac8f848654d42bc8af1087e
Signed-off-by: Abinaya P <abinayap@codeaurora.org>