phy: dphy: Add configuration helpers

The MIPI D-PHY spec defines default values and boundaries for most of the
parameters it defines. Introduce helpers to help drivers get meaningful
values based on their current parameters, and validate the boundaries of
these parameters if needed.

These helpers and header are taken from Linux commit 9123e3a74ec7 ("Linux 5.9-rc1").

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