scripts: dtc: only show unique unit address warning for enabled nodes

In some cases an hardware peripheral can be used for two exclusive usages.
For example, on STM32MP15 we have the same peripheral for I2S and SPI. We
have dedicated driver for each usage and so a dedicated device node in
devicetree.
To avoid to get useless warnings running "make W=1 dtbs", this patch adds
the "-Wunique_unit_address_if_enabled" flag for a make with W=1. In this
case we will detect a duplicate address only if both devices are
enabled in the devicetree, which is a real error case.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Link: https://lore.kernel.org/r/20221021084447.5550-1-alexandre.torgue@foss.st.com


[robh: Refactor options and keep 'unique_unit_address' for W=2]
Signed-off-by: Rob Herring <robh@kernel.org>