samples: subsys: llext: shell_loader: Exclude Renesas RZ/A2M

RZ/A2M requires MMU enabled. This sample disables MMU, so exclude the
platform.

Signed-off-by: Hoang Nguyen <hoang.nguyen.jx@bp.renesas.com>
Signed-off-by: Binh Nguyen <binh.nguyen.xw@renesas.com>