dts: flash: arm: nxp: Add write-block-size properties

On the various NXP Kinetis SoCs add the write-block-size property and
set it to match FSL_FEATURE_FLASH_PFLASH_BLOCK_WRITE_UNIT_SIZE for the
given SoC.

Fixes: #5788

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