ARM/dts: Set ref clock to 24MHz for msm8974

Set ref clock rate to 24MHz for 8974 platform for ref clock
based SOF/ITP generation. Otherwise enumeration of peripherals
will fail.

Change-Id: I8425e82f5ceaf3a8bcb4a5d1a3ccc7954e44d752
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>