ata: libahci: Allow using multiple regulators

The current implementation of the libahci allows using multiple PHYs
but not multiple regulators. This patch adds the support of multiple
regulators. Until now it was mandatory to have a PHY under a subnode,
now a port subnode can contain either a regulator or a PHY (or both).

In order to be able to asociate a port with a regulator the port are
now a platform device in the device tree case.

There was only one driver which used directly the regulator field of
the ahci_host_priv structure. To preserve the bisectability the change
in the ahci_imx driver was done in the same patch.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>