cmake: dts: Tweak warning flags for unit_address check

In Zephyr we have dts in which multiple peripherals can be at the same
MMIO address.  So we want to disable the `unique_unit_address` check via
(-Wno-unique_unit_address).  However, newer dtc support a check that
makes sure if we have duplicate unit-address devices that only one is
enabled.  So if the dtc flag is supported we enable
(-Wunique_unit_address_if_enabled).

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