[ARM] pxa/mioa701: add V3 gain configuration for Maxim 1586 voltage regulator

Maxim 1586 V3 gain can be configured by external resistors
(R24, R25) connected to the FB3 pin. To avoid accidental
overvoltage, it now requires this gain to be configured
explicitly.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.miao@marvell.com>