dts: flash: add missing DT properties to jedec,spi-nor

The erase-block-size and write-block-size are needed for the spi_nor
driver to compile.

scripts/dts/extract/flash.py automatically adds these properties if
this flash device is part of the "chosen" flash node, but they're
otherwise missing.

Signed-off-by: Jim Paris <jim@jtan.com>