ARM: dts: msm: reserve memory for splash on sdm670

Reserve memory for continuous splash feature on sdm670.
This memory buffer would be configured by the bootloader
and needs to be retained until the first frame update in
kernel.

Change-Id: I1db0e2538df5b83200eab3dd1a7472961e3e2a0d
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>