kontron-sl-mx8mm: Let CONFIG_SPL_FIT_IMAGE_TINY be selected

When CONFIG_IMX_HAB is selected the 'hab_status' command reports several
error events, indicating that the BootROM failed to authenticate the SPL.

After inspecting the content of the memory location that corresponds to
the DTB load address, the content did not match with the DTB binary,
showing that some kind of memory corruption/overlap occurred.

Letting the CONFIG_SPL_FIT_IMAGE_TINY option to be selected causes the
DTB to be properly placed into RAM and no more overlap occurs.

With this change, the 'hab_status' command returns no more error events,
which indicates that the BootROM succeeded to authenticate the SPL.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>