ARM: imx: fix typo "DEBUG_IMX50_IMX53_UART"

Commit f8c95fe6 (ARM: imx: support DEBUG_LL uart port selection for all
i.MX SoCs) had a typo that DEBUG_IMX50_IMX53_UART should be
DEBUG_IMX53_UART.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>