ARM: dts: msm: Add dev ref clk property for SDM660

This change add the device reference clock property
which sets the ref clk freq to 19.2 Mhz.

Change-Id: I6e8f5f57b40019556d15289ef91f390c17b50d11
Signed-off-by: Nitin Rawat <nitirawa@codeaurora.org>