dt-binding: phy: Add i.MX8MP PCIe PHY binding

Add i.MX8MP PCIe PHY binding.
On i.MX8MM, the initialized default value of PERST bit(BIT3) of
SRC_PCIEPHY_RCR is 1b'1.
But i.MX8MP has one inversed default value 1b'0 of PERST bit.

And the PERST bit should be kept 1b'1 after power and clocks are stable.
So add one more PERST explicitly for i.MX8MP PCIe PHY.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
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: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1665625622-20551-2-git-send-email-hongxing.zhu@nxp.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>