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>

drivers/spi/Kconfig

@@ -2,5 +2,11 @@ config DM_SPI
 	bool "Enable Driver Model for SPI drivers"
 	depends on DM
 	help
-	  If you want to use driver model for SPI drivers, say Y.
-	  To use legacy SPI drivers, say N.
+	  Enable driver model for SPI. The SPI slave interface
+	  (spi_setup_slave(), spi_xfer(), etc.) is then implemented by
+	  the SPI uclass. Drivers provide methods to access the SPI
+	  buses that they control. The uclass interface is defined in
+	  include/spi.h. The existing spi_slave structure is attached
+	  as 'parent data' to every slave on each bus. Slaves
+	  typically use driver-private data instead of extending the
+	  spi_slave structure.