PCI: imx6: Collect clock enables in imx6_pcie_clk_enable()

Encapsulate the i.MX PCIe clock enable operations into one standalone
function, imx6_pcie_clk_enable().  No functional change intended.

[bhelgaas: split pure code moves into separate patches]
Link: https://lore.kernel.org/r/1657783869-19194-7-git-send-email-hongxing.zhu@nxp.com
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>