ARM: dts: Configure PM8084 GPIO8 to output LPG4 on 8974 Pro AC MTP

PMA8084 GPIO8 on 8974 Pro AC MTP should be configured as special
function 1 output. This gpio drives the backlight control of the DSI
panel.

Change-Id: Ib5785d2a03d7fc3cc11475610830d6c0d7981d81
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>