HSI: omap_ssi: built omap_ssi and omap_ssi_port into one module

Merge omap_ssi and omap_ssi_port into one module. This
fixes problems with module cycle dependencies introduced
by future patches.

Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sre@kernel.org>