video: dw-mipi-dsi: driver-specific configuration of phy timings

The timing values for dw-dsi are often dependent on the used display and
according to Philippe Cornu will most likely also depend on the used phy
technology in the soc-specific implementation.

To solve this and allow specific implementations to define them as needed
add a new get_timing callback to phy_ops and call this from the dphy_timing
function to retrieve the necessary values for the specific mode.

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

[1] 25ed8aeb9c39 ("drm/bridge/synopsys: dsi: driver-specific configuration of phy timings")

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