sata_mv: remove iounmap in mv_platform_remove and use devm_iomap

this will fix crash bug when doing rmmod to the driver, this is because the
port_stop function get called later and it could access the device's registers.

Signed-off-by: Saeed Bishara <saeed@marvell.com>
Acked-by: Mark Lord <mlord@pobox.com>
Acked-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>