kbuild: turn of dtc unit address warnings by default

DTC 1.4.2 or later checks DT unit-address without reg property and
vice-versa, and generates lots of warnings.  Fixing DT files will
take for a while.  Until then, let's turn off the check unless
building with W=*.

Introduce a new helper dtc-option to check if the option is supported
in order to suppress warnings on older versions.

Signed-off-by: Masahiro Yamada <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Tested-by: Bin Meng <[email protected]>

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