reboot-mode: add support for reboot mode control

A new driver uclass is created to handle the reboot mode control.

The new uclass driver is updating an environment variable with the
configured reboot mode. The mode is extracted from a map provided
at initialization time. The map contains a list of modes
and associated ids.

Signed-off-by: Nandor Han <nandor.han@vaisala.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/Makefile b/drivers/Makefile
index 4081289..82d3c98 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -95,6 +95,7 @@
 obj-$(CONFIG_PCH) += pch/
 obj-y += phy/allwinner/
 obj-y += phy/marvell/
+obj-$(CONFIG_DM_REBOOT_MODE) += reboot-mode/
 obj-y += phy/rockchip/
 obj-y += phy/socionext/
 obj-y += rtc/