[ARM] pxa/hx4700: add Maxim 1587A voltage regulator

On this board, the PXA270 CPU voltage VCC_CORE is provided
by a Maxim 1587A voltage regulator configured to provide
1.55 V maximum voltage for 624 MHz operation.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Eric Miao <eric.miao@marvell.com>