scripts/Kconfig: add dt_node_array_prop Kconfig functions

The motivation is to allow accessing array elements from device tree
properties in Kconfig.
* Add dt_node_array_prop and _node_array_prop functions to extract the
array elements from properties.
* Add 'dt_node_array_prop_int' and 'dt_node_array_prop_hex' keys to use
in Kconfig.

Signed-off-by: Eivind Jølsgard <eivind.jolsgard@nordicsemi.no>