ARM: dts: msm: Shutdown the external-mdm before the internal

Due to the fact that external modem wouldn't survive without the
internal one, set the poff-depends-on flag for the internal modem.
This is to ensure that the external modem is always powered-off
before the internal modem.

Change-Id: I5399bef1fef1544dc883badd0f9660c0c91ebe8b
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>