soc: microchip: mpfs: Fix memory leak in mpfs_sys_controller_probe()

soc: microchip: mpfs: Fix memory leak in mpfs_sys_controller_probe()

In mpfs_sys_controller_probe(), if of_get_mtd_device_by_node() fails,
the function returns immediately without freeing the allocated memory
for sys_controller, leading to a memory leak.

Fix this by jumping to the out_free label to ensure the memory is
properly freed.

Also, consolidate the error handling for the mbox_request_channel()
failure case to use the same label.

Fixes: 742aa6c563d2 ("soc: microchip: mpfs: enable access to the system controller's flash")
Co-developed-by: Jianhao Xu <jianhao.xu@seu.edu.cn>
Signed-off-by: Jianhao Xu <jianhao.xu@seu.edu.cn>
Signed-off-by: Zilin Guan <zilin@seu.edu.cn>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>