phy: freescale: imx8m-pcie: Refine i.MX8MM PCIe PHY driver

To make it more flexible and easy to expand. Refine i.MX8MM PCIe PHY
driver.
- Use gpr compatible string to avoid the codes duplications when add
  another platform PCIe PHY support.
- Re-arrange the codes to let it more flexible and easy to expand.
No functional change. Re-arrange the TX tuning, since internal registers
can be wrote through APB interface before assertion of CMN_RST.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Marek Vasut <marex@denx.de>
Tested-by: Richard Leitner <richard.leitner@skidata.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.kernel.org/r/1665625622-20551-4-git-send-email-hongxing.zhu@nxp.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>