imx7: SPI: add suport for SPI flash in mikroBUS slot

Enable the escpi3 nets attached to the mikroBUS slot
on the i.MX7 Sabre evalution board. Also enble the SPI flash
commands to work with the "flash click" board.

This is V2 of this patch with changes recommended by the maintainer

CC: Jagan Teki <jteki@openedev.com>