ARM: dts: mxs: convert usb0_id_pins_a to use symbolic pin defs

This pin definition had been added after the initial patch to use
symbolic pin names in DTS files.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>