phy: marvell: a3700: Change return type of macro MVEBU_REG

All the calls to reg_set and friends have to cast the first argument
to void __iomem *. Lets change the return type of the MVEBU_REG macro
instead.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Signed-off-by: Stefan Roese <sr@denx.de>