drivers: regulator: add GPIO-controlled regulator driver

This provides structure for the regulator device hierarchy and a
driver for GPIO-controlled regulators along with its binding.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index eafe17c..3491113 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -99,4 +99,6 @@
 
 source "drivers/peci/Kconfig"
 
+source "drivers/regulator/Kconfig"
+
 endmenu