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/serial/Kconfig b/drivers/serial/Kconfig
index a0b6e02..c94353b 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -2,8 +2,10 @@
 	bool "Enable Driver Model for serial drivers"
 	depends on DM
 	help
-	  If you want to use driver model for serial drivers, say Y.
-	  To use legacy serial drivers, say N.
+	  Enable driver model for serial. This replaces
+	  drivers/serial/serial.c with the serial uclass, which
+	  implements serial_putc() etc. The uclass interface is
+	  defined in include/serial.h.
 
 config UNIPHIER_SERIAL
 	bool "UniPhier on-chip UART support"