ARM: dts: msm: adjust virtual key map for msm8909 skuc

The virtual keys KEY_MENU and KEY_BACK are incorrectly defined
for msm8909 skuc. Exchange the positions of these two virtual
keys for a correct operation of the device.

Change-Id: I2567863e163a7326d8b496c8ddc66039f894599e
Signed-off-by: Mao Li <maol@codeaurora.org>