MLK-21150-3 drm/bridge: sec-dsim: add a new property 'pref-rate'

MLK-21150-3 drm/bridge: sec-dsim: add a new property 'pref-rate'

Add a new property 'pref-rate' support which can be used to
assign a different clock frequency for the DPHY PLL reference
clock in the dtb file. And if this property does not exist,
the default clock frequency for the reference clock will be
used. And according to the spec, the DPHY PLL reference clk
frequency should be in [6MHz, 300MHz] range.

Signed-off-by: Fancy Fang <chen.fang@nxp.com>
(cherry picked from commit a9fafe8108505f8a1580af898ff5fa9c26d03680)