dt-bindings: bridge: samsung-dsim: Make some flags optional

In the event a device is connected to the samsung-dsim
controller that doesn't support the burst-clock, the
driver is able to get the requested pixel clock from the
attached device or bridge.  In these instances, the
samsung,burst-clock-frequency isn't needed, so remove
it from the required list.

The pll-clock frequency can be set by the device tree entry
for samsung,pll-clock-frequency, but in some cases, the
pll-clock may have the same clock rate as sclk_mipi clock.
If they are equal, this flag is not needed since the driver
will use the sclk_mipi rate as a fallback.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230526030559.326566-8-aford173@gmail.com