[ARM] 2971/1: i.MX uart handle rts irq

Patch from Sascha Hauer

handle rts interrupt

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