drivers: mipi_dsi: implement clock selection algorithm

With the phy-clock being specified in devicetree (and thus under user
control), there is no need to artificially enlarge the DPHY clock to
insure it is fast enough. Instead, we can calculate the DPHY clock
directly, selecting the closest realizable value that is at least as
fast as the value requested by the user.

Fixes #59215

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>