ARM: mach-mx5/mx53_ard: Add missing definition

commit 9203d59 (ARM:mach-mx5/mx53_ard: Add I2C2 and I2C3 support) missed to include
the define for ARD_I2CPORTEXP_B.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>