ata: sata_mv: add proper definitions for LP_PHY_CTL register values

Commit 9013d64e ("ata: sata_mv: fix disk hotplug for Armada
370/XP SoCs") added some manipulation of the LP_PHY_CTL register, but
using magic values. This commit changes the code to use proper
definitions for the LP_PHY_CTL register, which allows to document what
the different bits are doing.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Tejun Heo <tj@kernel.org>