usb: ehci-mx6: add support for otg port

Previously, only host1 was supported using an index of 0.
Now, otg has index 0, host1 is 1, host2 is 2, host3 is 3.
Since OTG requires usbmode to be set after reset, I added
CONFIG_EHCI_HCD_INIT_AFTER_RESET to nitrogen6x.h and
mx6qsabreauto.h.

I also added a weak function board_ehci_power to handle
turning power on/off for otg.

Type is type of device connected (USB stick vs Host.)
Init is type of device desired.
Only power up port if type == init == USB_INIT_HOST.
Only return error if type != init.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>