scripts: dts: extract: Fix handling of prop being a list of lists

Before dtc 1.4.7 we'd get something like the following for an gpios
property:

reg = <1 2 3 4>;

After dtc 1.4.7 we get:

cs-gpios = <0x05 0x0d 0x00>, < 0x06 0x00 0x00>;

We should handle both cases in the extract reg handling code.  So if
we see a list of lists, we flatten it to a single list to normalize
the property.

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