dt-bindings: Document correct and deprecated vendor-prefix with device isl29028

This patch documents the device isl29028 with its vendor-prefix. Undocumented deprecated vendor-prefix
found by checkpatch also documented for compatibility reasons.

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>