drm: bridge: samsung-dsim: Fetch pll-clock-frequency automatically

Make the pll-clock-frequency optional.  If it's present, use it
to maintain backwards compatibility with existing hardware.  If it
is absent, read clock rate of "sclk_mipi" to determine the rate.
Since it can be optional, change the message from an error to
dev_info.

Signed-off-by: Adam Ford <aford173@gmail.com>
Tested-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> # imx8mm-icore
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230526030559.326566-4-aford173@gmail.com