video:da8xx-fb: calculate pixel clock period for the panel

Patch calculates pixel clock period in pico seconds and updates
the same in variable screen information structure. fbset utility
uses this information.

Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>