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/gpio/Kconfig b/drivers/gpio/Kconfig
index d21302f..b609e73 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -2,5 +2,8 @@
 	bool "Enable Driver Model for GPIO drivers"
 	depends on DM
 	help
-	  If you want to use driver model for GPIO drivers, say Y.
-	  To use legacy GPIO drivers, say N.
+	  Enable driver model for GPIO access. The standard GPIO
+	  interface (gpio_get_value(), etc.) is then implemented by
+	  the GPIO uclass. Drivers provide methods to query the
+	  particular GPIOs that they provide. The uclass interface
+	  is defined in include/asm-generic/gpio.h.