Enable pcie support in uboot on imx6sx sd boards
- enable_pcie_clock should be call before ssp_en is set,
  since that ssp_en control the phy_ref clk gate, turn on
  it after the source of the pcie clks are stable.
- add debug info
- add rx_eq of gpr12 on imx6sx
- there are random link down issue on imx6sx. It's
  pcie ep reset issue.
  solution:reset ep, then retry link can fix it.

(cherry picked from commit ec78595a)
Signed-off-by: Richard Zhu <r65037@freescale.com>
Signed-off-by: Ye Li <ye.li@nxp.com>