imx: imx8: ahab: refactor do_ahab_close command

Move an OEM closing logic to ahab_close() function to be able to use
it directly without calling a u-boot command.

Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Co-developed-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>