cmd: add ADC cli commands

Add an 'adc' cli command to get information from adc devices and to read
"single shot" data.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/cmd/Makefile b/cmd/Makefile
index d734902..0d7322e 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -12,6 +12,7 @@
 
 # command
 obj-$(CONFIG_CMD_AES) += aes.o
+obj-$(CONFIG_CMD_ADC) += adc.o
 obj-$(CONFIG_CMD_ARMFLASH) += armflash.o
 obj-y += blk_common.o
 obj-$(CONFIG_SOURCE) += source.o