video: dw-mipi-dsi: permit configuring the escape clock rate

The Amlogic D-PHY in the Amlogic AXG SoC Family does support a frequency
higher than 10MHz for the TX Escape Clock, thus make the target rate
configurable.

This is based on the Linux commit [1] and adapted to the U-Boot driver.

[1] a328ca7e4af3 ("drm/bridge: dw-mipi-dsi: permit configuring the escape clock rate")

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>