doc: board: verdin-imx8mm: use mainline TF-A

1. Update build steps where mainline Trusted Firmware A is used.
2. Fix BL31_BASE to the proper one according to the SoC reference
manual.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>