dts: flash_map: Automatically generate of the list of flash area

This Patch add functionality for automatic generation of the flash map
using DTS description. Automatic generation allows to replace
C-hardcoded flash_map.

We generate a set of defines based on the index of a partiion:
	#define DT_FLASH_AREA_<IDX>_OFFSET	0
	#define DT_FLASH_AREA_<IDX>_SIZE	131072
	#define DT_FLASH_AREA_<IDX>_DEV		"FLASH_CTRL"
	#define DT_FLASH_AREA_<IDX>_LABEL	MCUBOOT

Additionally we also define:
	#define DT_FLASH_AREA_NUM		4

and:
	#define DT_FLASH_AREA_<PARTNAME>_ID	0

Signed-off-by: Findlay Feng <i@fengch.me>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>