doc: sl-mx8mm: Fix mistake in merge conflict resolution

doc: sl-mx8mm: Fix mistake in merge conflict resolution

There was a conflict between the following two commits, that wasn't
resolved correctly. Fix this.

6f118fd31a54 ("doc: sl-mx8mm: Update the NXP TF-A source reference")
b6eda67b523e ("imx: Suggest the NXP ATF github repo")

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Cc: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>