usb: host: fotg2: add device tree probing

Add device tree probing to the fotg2 driver.

Signed-off-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>
[Drop DMA mask coercion, drivers/of/platform.c does the job]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>