dm: add entries to Kconfig

Create entries of CONFIG_DM, CONFIG_DM_SERIAL, CONFIG_DM_GPIO
and CONFIG_DM_SPI.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>

drivers/core/Kconfig

+config DM
+	bool "Enable Driver Model"
+	depends on !SPL_BUILD
+	help
+	  This config option enables Driver Model.
+	  To use legacy drivers, say N.

drivers/gpio/Kconfig

+config DM_GPIO
+	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.

drivers/serial/Kconfig

+config DM_SERIAL
+	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.

drivers/spi/Kconfig

+config DM_SPI
+	bool "Enable Driver Model for SPI drivers"
+	depends on DM
+	help
+	  If you want to use driver model for SPI drivers, say Y.
+	  To use legacy SPI drivers, say N.