esoc: Fix bugs in esoc_client registration

Fix incorrect esoc name descriptor from subsys_esocn
to esocn.
Add stub definition for registration and unregister
in case CONFIG_ESOC_CLIENT is not set.
Force probe deferral if requested external soc
has not yet probed.

Change-Id: I10a19c9f5eb0a1483b8ebf930e67f94a70c76151
Signed-off-by: Hanumant Singh <hanumant@codeaurora.org>