HSI: export method to (un)register clients

Expose method for registering and unregistering HSI clients, so that
client drivers can register other client drivers.

This is useful for HSI drivers, which want to use the functionality
of other HSI drivers. For example the N900 modem driver can load HSI
drivers for mcsaab protocol and speech protocol.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Tested-By: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>