serial/imx: fix IMX UART macro usage to reflect correct processor

Platform dependant UART data refers to MX31 macro for MX35 machines.
For all other machines, macro usage matches machine type.

Though this compiles out to the same result, it looks much like
a typo, so fix it to use the right macros instead.

Signed-off-by: Oskar Schirmer <oskar@scara.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>