dts: flash: Add support for flash labels

Added a label for soc-nv-flash nodes.  Made some updates to the
generation to maintain creating defines for properties like
erase-block-size and write-block-size so they we get both
FLASH_ERASE_BLOCK_SIZE and FLASH_${LABEL}_ERASE_BLOCK_SIZE (same for
WRITE_BLOCK_SIZE).

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