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>