drm/bridge: tc358768: Enable reference clock

The driver assumes that reference clock is always-enabled, but this is
hardware-dependent and not true for ASUS Transformer TF700T. Make driver
enable / disable the reference clock.

Tested-by: Andreas Westman Dorcsak <hedmoo@yahoo.com> # Asus TF700T
Tested-by: Maxim Schwalm <maxim.schwalm@gmail.com> #TF700T
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211002233447.1105-2-digetx@gmail.com