[ARM] mx31ads: Add missing include

mx31ads_defconfig compilation failed with

arch/arm/mach-mx3/mx31ads.c: In function 'mxc_init_imx_uart':
arch/arm/mach-mx3/mx31ads.c:102: error: 'mxc_uart_device0' undeclared (first use in this function)
arch/arm/mach-mx3/mx31ads.c:102: error: (Each undeclared identifier is reported only once
arch/arm/mach-mx3/mx31ads.c:102: error: for each function it appears in.)
make[1]: *** [arch/arm/mach-mx3/mx31ads.o] Error 1

Add missing include

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>