[i.MX8] Add PWMs definition in dtsi file

This add-on regards the eight PWMs prensent into the LSIO part of the processor. This is due to the fact that in this version of kernel NXP has not included the nodes about these PWMs.
As reference the past kernel 4.19 (SECO internal) has been used.

Now it is possible to enable and use a pwm# node into dts of out boards.