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/thermal/Kconfig b/drivers/thermal/Kconfig
new file mode 100644
index 0000000..3c6b36d
--- /dev/null
+++ b/drivers/thermal/Kconfig
@@ -0,0 +1,7 @@
+config DM_THERMAL
+	bool "Driver support for thermal devices"
+	help
+	  Enable support for temporary-sensing devices. Some SoCs have on-chip
+	  temperature sensors to permit warnings, speed throttling or even
+	  automatic power-off when the temperature gets too high or low. Other
+	  devices may be discrete but connected on a suitable bus.