usb: misc: usb3503: Support operation with no I2C control

Refactor so that register writes for configuration are only performed if
the device has a regmap provided and also register as a platform driver.
This allows the driver to be used to manage GPIO based control of the
device.

Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: devicetree@vger.kernel.org
Reviewed-by: Dongjin Kim <tobetter@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>