[ARM] 2958/1: fix definition in imx-regs.h

Patch from Sascha Hauer

Fix PD7_AF_UART2_DTR definition

Signed-off-by: Giancarlo Formicuccia <gformicuccia@atinno.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>