phy: dphy: refactor get_default_config

Factor out the calculation into phy_mipi_dphy_calc_config(). This is
needed for the follow up patch which adds the support to calculate the
timings based on a given hs clock. No functional changes are done.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>