scripts/dts/extract/flash.py: fix load offset

When the code partition is not at the beginning of the flash, the load
offset is wrongly computed. The address in the device tree is already
relative to the beginning of the parent node, ie the beginning of the
flash memory space. There is therefore no need to subtract it.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>