bsp: Replace CONFIG_AUTOFS4_FS to fix config warning on Corstone-1000

CONFIG_AUTOFS4_FS is deprecated and should be replaced by
CONFIG_AUTOFS_FS.

In theory, this is only required for K3s, and K3s is not supported on
Corstone-1000. However, in their current form, the kernel settings are
not split split between K3s and docker.

Update the setting to resolve the warning in the short term.

In the long term this should be refactored so docker and K3s settings
can be specified separately - but that is not a priority now.

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