ARM: dts: msm: reserve cont-splash buffer memory for msm8992

Reserve memory for continuous splash screen feature
to avoid any kernel module from overwriting it during
boot up. This memory will be released by MDSS driver
when it receives first update from surface flinger.

Change-Id: I20af0eec6cf906a51202e9ff1cc440eb2f634d99
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>