ARM: dts: msm: adjust virtual key map for msm8996 dtp

The virtual keys KEY_MENU/HOME/KEY_BACK are incorrectly defined
for msm8996 dtp. Exchange the positions of these three virtual
keys for a correct operation of the device.

Change-Id: Ifd2a00291a9fcb195ac270ea85fd134e82ff5082
Signed-off-by: Mao Li <maol@codeaurora.org>