MLK-14938-17 pcie: Add support for i.MX8QM/QXP PCIe

MLK-14938-17 pcie: Add support for i.MX8QM/QXP PCIe

- one lane pcie gen2 link is okay, the cfg space
  of the rc/ep can be accessed.
  rc cfg base 0x5f00_0000. ep cfg base 0x6000_0000
- limit to gen2 speed
- mask the wait of eq3 finish, because it is used
  for gen3.
- use pcie_ctrla_init_rc() to do the initialization
  of the pciea controller
- setup the common pcie codes in pcie_imx8x.c, separate
  the different soc speicifed initialization codes into
  their own pcie/board codes, move the macro definitions
  into the new header file imx8_hsio.h.
- i.MX8QXP only have PCIe Control B. Enable PORT B at default.
  i.MX8QM needs to set CONFIG_IMX_PCIEB to enable PORT B.
- Disable the LTSSM when link is down.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 03141c2b955ce6034f06e701126aea1493dc2b4b)
(cherry picked from commit 99c90ff2de4849aafa0043932353e2c199d22e5f)