colibri_vf: Enable board specific USB initialisation for USB pen gpio

Add IOMUX for the pad used as USB pen. This needs to be driven low for
the Iris and Viola boards where it is pulled up high by default. This is
required for the USB host functionality to work on these boards. Use the
board specific weak initialisation function, to drive the pin low which
would be called on "usb start".

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>