phy: dphy: Clarify lanes parameter documentation

The lanes parameter is not solely about the number of lanes, but it also
carries the fact that those are the first lanes in use during the
transmission.

It was implicit so far, so make sure it's explicit now.

Suggested-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>