PCI: imx6: Fix IMX6SX_GPR12_PCIE_TEST_POWERDOWN handling

PCI: imx6: Fix IMX6SX_GPR12_PCIE_TEST_POWERDOWN handling

The IMX6SX_GPR12_PCIE_TEST_POWERDOWN bit does not control the PCIe
reference clock on i.MX6SX. Instead, it is part of i.MX6SX PCIe core
reset sequence.

Move the IMX6SX_GPR12_PCIE_TEST_POWERDOWN assertion/deassertion into
the core reset functions to properly reflect its purpose. Remove the
.enable_ref_clk() callback for i.MX6SX since it was incorrectly
manipulating this bit.

Fixes: e3c06cd063d6 ("PCI: imx6: Add initial imx6sx support")
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260319090844.444987-1-hongxing.zhu@nxp.com