scripts/dts/extract: fix handling of non-MMIO SPI flash

The Intel S1000 board has a SPI flash that isn't directly MMIO
addressable.  For this case we shouldn't generate CONFIG_FLASH_SIZE or
CONFIG_FLASH_BASE_ADDRESS.

We use the heuristic of assuming if the flash node #size-cells is 0, its
a SPI flash.  Than if the parent node (SPI controller) has only a single
reg element we assume its a non-MMIO addressable SPI flash.  If there is
more than one reg element we assume the last reg element is the MMIO
address region for access to the flash.

Fixes #12530

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