iio/adc: ingenic: Fix battery VREF for JZ4770 SoC

The reference voltage for the battery is clearly marked as 1.2V in the
programming manual. With this fixed, the battery channel now returns
correct values.

Fixes: a515d648 ("IIO: Ingenic JZ47xx: Add support for JZ4770 SoC ADC.")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Artur Rojek <contact@artur-rojek.eu>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20201104192843.67187-1-paul@crapouillou.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>