ARM: dts: msm: Add default bootdevice as ufs for SM6150

If eMMC is added as default bootdevice, as per
the current implementation, bootloader is unable
to overwrite the bootdevice address after boot up
(incase if we need to mount ufs)

Update default bootdevice as ufs(1d84000.ufshc)
so that bootloader can overwrite the bootdevice.

Change-Id: Iabe2797f37ea9ac1a050d7b307e523c5fabccf89
Signed-off-by: Kiran Raparthy <kraparth@codeaurora.org>