sata_mv: implement SoC guideline SATA_S11

The 5182 System-On-Chip (SOC) variant wants certain lower
bits to be cleared on any write to the PHY_MODE3 register.

If/when support is added for other SOC variants, we'll need
some way to uniquely identify the 5182, and not perform this
workaround for the others.

But for now, it is the only SOC variant we support here.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>