doc: Relax micro SD card requirement for Corstone-1000

There does not appear to be any specific size requirement for the micro
SD card used with the Arm Corstone-1000 for MPS3. Relax the wording to
allow any card which is `at least 4 GB` as cards which are exactly 4 GB
may now be difficult to source.

Signed-off-by: Adam Johnston <adam.johnston@arm.com>