HSI: Introduce Nokia N900 modem driver

The Nokia N900's modem is connected via Synchronous Serial Interface (SSI),
which is a legacy version of MIPI's High-speed Synchronous Serial Interface
(HSI).

The handles the GPIOs for enabling and resetting the modem and instanciates
ssi-protocol for data exchange. It does not yet support exchanging voice data
with the modem.

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>
diff --git a/drivers/hsi/clients/Kconfig b/drivers/hsi/clients/Kconfig
index 1457cfb..71b9f9a 100644
--- a/drivers/hsi/clients/Kconfig
+++ b/drivers/hsi/clients/Kconfig
@@ -4,6 +4,15 @@
 
 comment "HSI clients"
 
+config NOKIA_MODEM
+	tristate "Nokia Modem"
+	depends on HSI && SSI_PROTOCOL
+	help
+	Say Y here if you want to add support for the modem on Nokia
+	N900 (Nokia RX-51) hardware.
+
+	If unsure, say N.
+
 config SSI_PROTOCOL
 	tristate "SSI protocol"
 	depends on HSI && PHONET && (OMAP_SSI=y || OMAP_SSI=m)