dm: Expand and complete Kconfig in drivers/

Expand the help messages for each driver. Add missing Kconfig for I2C,
SPI flash and thermal.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/drivers/core/Kconfig b/drivers/core/Kconfig
index d2799dc..dc32385 100644
--- a/drivers/core/Kconfig
+++ b/drivers/core/Kconfig
@@ -2,5 +2,7 @@
 	bool "Enable Driver Model"
 	depends on !SPL_BUILD
 	help
-	  This config option enables Driver Model.
-	  To use legacy drivers, say N.
+	  This config option enables Driver Model. This brings in the core
+	  support, including scanning of platform data on start-up. If
+	  CONFIG_OF_CONTROL is enabled, the device tree will be scanned also
+	  when available.