i2c: Prevent priority inversion on top of bus lock

Low priority thread holding the i2c bus mutex could block higher
priority threads to access the bus resulting in unacceptable
latencies. Change the mutex type to rt_mutex preventing priority
inversion.

Tested-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@nokia.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>

drivers/i2c/Kconfig

@@ -5,6 +5,7 @@
 menuconfig I2C
 	tristate "I2C support"
 	depends on HAS_IOMEM
+	select RT_MUTEXES
 	---help---
 	  I2C (pronounce: I-square-C) is a slow serial bus protocol used in
 	  many micro controller applications and developed by Philips.  SMBus,