USB: MX5: Add MX5 usb post-init callback

This is useful for USB Transceivers init etc.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Remy Bohmer <linux@bohmer.net>
Cc: Wolfgang Grandegger <wg@denx.de>
Cc: Jason Liu <r64343@freescale.com>