HSI: Introduce OMAP SSI driver

Add OMAP SSI driver to the HSI subsystem.

The Synchronous Serial Interface (SSI) is a legacy version
of HSI. As in the case of HSI, it is mainly used to connect
Application engines (APE) with cellular modem engines (CMT)
in cellular handsets.

It provides a multichannel, full-duplex, multi-core communication
with no reference clock. The OMAP SSI block is capable of reaching
speeds of 110 Mbit/s.

Signed-off-by: Carlos Chinea <carlos.chinea@nokia.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Tested-By: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
diff --git a/drivers/hsi/Kconfig b/drivers/hsi/Kconfig
index d94e38d..2c76de4 100644
--- a/drivers/hsi/Kconfig
+++ b/drivers/hsi/Kconfig
@@ -14,6 +14,7 @@
 	bool
 	default y
 
+source "drivers/hsi/controllers/Kconfig"
 source "drivers/hsi/clients/Kconfig"
 
 endif # HSI