mx6cuboxi: Skip usb initialization when booting without HDMI

Starting USB initialization is useful for those who use Cuboxi/Hummingboard
with HDMI and USB keyboard.

However, when booting without a HDMI connection we can skip the usb
initialization, which makes the boot faster.

Signed-off-by: Jon Nettleton <jon.nettleton@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-By: Vagrant Cascadian <vagrant@aikidev.net>