flash: Convert DT_FLASH_AREA to FLASH_AREA macros

Convert with a combo of scripts and by hand fixups:

git grep -l DT_FLASH_AREA_.*_ID | \
 xargs sed -i -r 's/DT_FLASH_AREA_(.*)_ID/FLASH_AREA_ID(\L\1)/'

git grep -l DT_FLASH_AREA_.*_OFFSET | \
 xargs sed -i -r 's/DT_FLASH_AREA_(.*)_OFFSET/FLASH_AREA_OFFSET(\L\1)/'

git grep -l DT_FLASH_AREA_.*_SIZE | \
 xargs sed -i -r 's/DT_FLASH_AREA_(.*)_SIZE/FLASH_AREA_SIZE(\L\1)/'

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>