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/mtd/Kconfig b/drivers/mtd/Kconfig
index 415ab4e..59278d1 100644
--- a/drivers/mtd/Kconfig
+++ b/drivers/mtd/Kconfig
@@ -1 +1,3 @@
 source "drivers/mtd/nand/Kconfig"
+
+source "drivers/mtd/spi/Kconfig"