PCI: imx: Fix probe failure without power domain

On chips without a separate power domain for PCI (such as 6q/6qp) the
imx6_pcie_attach_pd() function incorrectly returns an error.

Fix by returning 0 if dev_pm_domain_attach_by_name() does not find
anything.

Fixes: 3f7cceea ("PCI: imx: Add multi-pd support")
Reported-by: Lukas F.Hartmann <lukas@mntmn.com>
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>