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/i2c/Kconfig b/drivers/i2c/Kconfig
index 202ea5d..2cc776c 100644
--- a/drivers/i2c/Kconfig
+++ b/drivers/i2c/Kconfig
@@ -2,8 +2,16 @@
 	bool "Enable Driver Model for I2C drivers"
 	depends on DM
 	help
-	  If you want to use driver model for I2C drivers, say Y.
-	  To use legacy I2C drivers, say N.
+	  Enable driver model for I2C. This SPI flash interface
+	  (spi_flash_probe(), spi_flash_write(), etc.) is then
+	  implemented by the SPI flash uclass. There is one standard
+	  SPI flash driver which knows how to probe most chips
+	  supported by U-Boot. The uclass interface is defined in
+	  include/spi_flash.h, but is currently fully compatible
+	  with the old interface to avoid confusion and duplication
+	  during the transition parent. SPI and SPI flash must be
+	  enabled together (it is not possible to use driver model
+	  for one and not the other).
 
 config SYS_I2C_UNIPHIER
 	bool "UniPhier I2C driver"