ARM / highbank: add support for pl320 IPC

The pl320 IPC allows for interprocessor communication between the
highbank A9 and the EnergyCore Management Engine. The pl320 implements
a straightforward mailbox protocol.

Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index f5fb072..2b4e89b 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -134,6 +134,8 @@
 
 source "drivers/clocksource/Kconfig"
 
+source "drivers/mailbox/Kconfig"
+
 source "drivers/iommu/Kconfig"
 
 source "drivers/remoteproc/Kconfig"