Add support for the battery voltage measurement part of the JZ4740 ADC unit.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/1416/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>