usb: misc: usb3503: Add USB3503A to the compatible list

There are no software visible differences that I am aware of but in case
any are discovered allow the DTS to specify exactly which device is
present.

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>