kbuild: fix W= option checks for extra DTC warnings

Kbuild supports 3 levels of extra warnings, and multiple levels can
be combined, like W=12, W=123.  It was added by commit a6de553d
("kbuild: Allow to combine multiple W= levels").

From the log of commit 8654cb8d

 ("dtc: update warning settings
for new bus and node/property name checks"), I assume:

 - unit_address_vs_reg, simple_bus_reg, etc. belong to level 1
 - node_name_chars_strict, property_name_chars_strict belong to level 2

However, the level 1 warnings are displayed by any argument to W=.
On the other hand, the level 2 warnings are displayed by W=2, but
not by W=12, or W=123.

Use $(findstring ...) like scripts/Makefile.extrawarn.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>