plat: imx8mm: provide uart base as build option

Some boards (f.e. Verdin i.MX8M Mini) use different UART base address
for serial debug output, so make this value configurable (as a
build option).

Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Change-Id: I988492ccecbc3f64a5153b381c4a97b8a0181f52
2 files changed