video: mxsfb: add enabling of "axi" clock other than "per" clock

video: mxsfb: add enabling of "axi" clock other than "per" clock

On some SoC mxsfb needs more than one clock gate(actual "per" clock). So
let's introduce "axi" clock that can be provided but it's not mandatory.
This is inspired from linux mxsfb driver. Also let's rename "per" clock to
"pix" clock for compatibility with already existing .dts lcdif nodes
implementation.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>