ARM: dts: msm: Early mount of vendor partition for SDM670

Add support to early mount vendor partition from device tree. This is
required so that vendor modules can be loaded during early-init.

Change-Id: I2f420a1ff78d5edead77a90f551972f451edae3e
Signed-off-by: Imran Khan <kimran@codeaurora.org>
Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>