flash: dts: replace DT_FLASH_{ERASE,WRITE}_BLOCK_SIZE with new macros

Replace:
	DT_FLASH_ERASE_BLOCK_SIZE ->
	DT_PROP(DT_CHOSEN(zephyr_flash), erase_block_size)

	DT_FLASH_WRITE_BLOCK_SIZE ->
	DT_PROP(DT_CHOSEN(zephyr_flash), write_block_size)

As this allows us to phase out the old generator.

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