plat: imx: imx8qm: provide debug uart num as build param

This removes hardcoded iomux/clk/addr configuration for debug uart,
provides possibility (as a workaround, till that information isn't
provided via DT) to set this configuration during compile time via
IMX_DEBUG_UART build flag.

Usage:
$ make PLAT=imx8qm IMX_DEBUG_UART=1 bl31

Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Change-Id: Ib5f5dd81ba0c8ad2b2dc5647ec75629072f511c5