kbuild: Don't report disabled nodes with duplicate addresses

Duplicated unit address is okay if we have only one such node enabled.
So, remove '-Wno-unique_unit_address' from DTC_FLAGS.

This helps in reducing warnings in qcom dts from

   6483 unique_unit_address
   1108 simple_bus_reg
    764 avoid_unnecessary_addr_size
    712 unit_address_vs_reg
    120 graph_child_address
     32 unique_unit_address_if_enabled

to:
    277 simple_bus_reg
    191 avoid_unnecessary_addr_size
    178 unit_address_vs_reg
     32 unique_unit_address_if_enabled
     30 graph_child_address

which would help people focus on the actual warnings and fix them.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>