scripts/write_flash.sh: add an offest into the flash

The DDR firmware will need to be written somewhere other than offset 0

Signed-off-by: Angus Ainslie <angus@akkea.ca>
3 jobs for ddr-fw in 3 minutes and 11 seconds
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librem5
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librem5
test:download:deb

00:01:30

passed #303101
librem5
test:download:pip

00:01:53