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/Kconfig b/drivers/Kconfig
index 021293e..dcce532 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -50,4 +50,6 @@
 
 source "drivers/crypto/Kconfig"
 
+source "drivers/thermal/Kconfig"
+
 endmenu