HID: intel-ish-hid: Move driver registry functions

Move the driver registry with the ishtp bus to the common interface
file, which clients can include.
Also rename __ishtp_cl_driver_register() to ishtp_cl_driver_register()
and removed define for ishtp_cl_driver_register.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>