dm: Add support for simple-pm-bus

This type of bus is used in Linux to designate buses which have power
domains and/or clocks which need to be enabled before their child devices
can be used. Because power domains are automatically enabled before probing
in U-Boot, we just need to enable any clocks present.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 20a81a0..692432c 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -96,6 +96,7 @@
 CONFIG_SYSCON=y
 CONFIG_DEVRES=y
 CONFIG_DEBUG_DEVRES=y
+CONFIG_SIMPLE_PM_BUS=y
 CONFIG_ADC=y
 CONFIG_ADC_SANDBOX=y
 CONFIG_AXI=y