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/mtd/Kconfig

@@ -1 +1,3 @@
 source "drivers/mtd/nand/Kconfig"
+
+source "drivers/mtd/spi/Kconfig"

drivers/mtd/spi/Kconfig

@@ -0,0 +1,14 @@
+config DM_SPI_FLASH
+	bool "Enable Driver Model for SPI flash"
+	depends on DM && SPI
+	help
+	  Enable driver model for SPI flash. 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).