ar71xx: Add OTP support for AR934x

The patch 523-MIPS-ath79-OTP-support only supports the OTP offsets for
AR933x chips, which has changed on newer platforms such as the AR934x.
The follwoing change is to add support for reading the OTP on the
AR934x. Tested on an Aerohive AP-121.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Chris Blake <chrisrblake93@gmail.com>