PCI: imx6: Mark PHY functions as i.MX6 specific

PCIe PHY IP block on i.MX7D differs from the one used on i.MX6 family,
so none of the code in the current implementation of
imx6_setup_phy_mpll() or imx6_pcie_reset_phy() is applicable.

Introduce IMX6_PCIE_FLAG_IMX6_PHY and check for it in the aforementioned
functions to make sure they are only executed on appropriate PCIe IP
variants.

Tested-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
[lorenzo.pieralisi@arm.com: updated log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Leonard Crestez <leonard.crestez@nxp.com>
Cc: "A.s. Dong" <aisheng.dong@nxp.com>
Cc: Richard Zhu <hongxing.zhu@nxp.com>