dm: extcon: add an uclass for extcon

Add a new simple uclass for extcon. Currently all setup is done
in the probe. Uclass struct and ops are empty for now.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/Makefile b/drivers/Makefile
index 58be410..ed1e71c 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -9,6 +9,7 @@
 obj-$(CONFIG_$(SPL_TPL_)DMA) += dma/
 obj-$(CONFIG_$(SPL_TPL_)DMA_LEGACY) += dma/
 obj-$(CONFIG_$(SPL_TPL_)DFU) += dfu/
+obj-$(CONFIG_$(SPL_TPL_)EXTCON) += extcon/
 obj-$(CONFIG_$(SPL_TPL_)GPIO) += gpio/
 obj-$(CONFIG_$(SPL_TPL_)DRIVERS_MISC) += misc/
 obj-$(CONFIG_$(SPL_TPL_)SYSRESET) += sysreset/