include/devicetree.h: Add DT_STRING_TOKEN_OR macro

This macro expands to DT_STRING_TOKEN if property exists, otherwise
falls back to default value.
Helpful when a non-required enum binding doesn't mention a default
value, but a default value makes sense to be set in the code.

Including DT_INST_STRING_TOKEN_OR and test code.

Signed-off-by: Bernardo Perez Priego <bernardo.perez.priego@intel.com>