dt-bindings: Document deprecated device vendor name to fix related warning

This patch documents deprecated vendor name "isil" to fix warning of
undocumented string for device isl29028 as reported while running checkpatch.pl
on drivers/staging/iio/light/isl29028.c. This is done to maintain compatibility
with older kernels.
Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -77,6 +77,7 @@ innolux	Innolux Corporation
 intel	Intel Corporation
 intercontrol	Inter Control Group
 isee	ISEE 2007 S.L.
+isil    Intersil (deprecated, use isl)
 isl	Intersil
 karo	Ka-Ro electronics GmbH
 keymile	Keymile GmbH