PCI: imx6: Factor out ref clock disable to match enable

PCI: imx6: Factor out ref clock disable to match enable

The PCIe ref clocks are specific to different variants.  The enables are
already split out into imx6_pcie_enable_ref_clk(), but the disables were
combined with the more generic bus/phy/pcie clock disables in
imx6_pcie_clk_disable().

Split out the variant-specific disables into imx6_pcie_disable_ref_clk() to
match imx6_pcie_enable_ref_clk().

No functional change intended.

Link: https://lore.kernel.org/r/1657783869-19194-6-git-send-email-hongxing.zhu@nxp.com
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Richard Zhu <hongxing.zhu@nxp.com>