scripts/dts/extract: Support QSPI memory mapped flash

We need to refactor how we determine CONFIG_FLASH_{SIZE,BASE_ADDRESS}
for when we have a QSPI that is memory mapped. "zephyr,flash" is going
to point the actual flash component that will be on a SPI bus and thus
the device node's reg property tells us which CS on the SPI bus its on,
not the memory mapped address/size of the flash.

So we make a few assumptions to handle this case:
1. If the #size-cells for the flash node is 0, we assume its a QSPI
   memory mapped flash
2. That the QSPI memory mapped node (parent of what "zephyr,flash" is
   point to, will contain a reg propery where the second reg pair will
   be the memory mapped region for the flash.

We move handling of CONFIG_FLASH_{SIZE,BASE_ADDRESS} into flash.extract
so its all in one place.

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