scripts/Kconfig: add dt_node_str_prop_equals Kconfig function

Add function to check if a property of a node
is equal to a passed string.

The motivation is that in current USB device support
the sizes of bulk endpoints are configured in Kconfig.
The information if a USB device controller supports
high-speed comes from devicetree. This information must be
mapped in controller driver Kconfig and corresponding options
in USB device stack configured must be adjusted.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>