of: unittest: document intentional interrupt-map provider build warning

of: unittest: document intentional interrupt-map provider build warning

A recently implemented dtc compiler warning reports a dts problem
via a build warning:

   drivers/of/unittest-data/tests-interrupts.dtsi:32.26-35.6: Warning (interrupt_map): /testcase-data/interrupts/intmap1: Missing '#address-cells' in interrupt-map provider

The warning will be addressed by a separate patch by suppressing the
warning for .dts files that include this .dtsi.  This patch documents
why the warning is due to a deliberately incorrect .dtsi file so that
no one will fix the .dtsi file to prevent the build warning.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Link: https://lore.kernel.org/r/20211030011039.2106946-1-frowand.list@gmail.com
Signed-off-by: Rob Herring <robh@kernel.org>