arm64: dts: ti: k3-am68-sk: Add DT node for PCIe

AM68 Starter kit features with one PCIe M.2 Key M connector
interfaced via two SerDes lanes. Update the SerDes configuration
for PCIe.

Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>
Signed-off-by: Ravi Gunasekaran <r-gunasekaran@ti.com>
Link: https://lore.kernel.org/r/20230921100039.19897-3-r-gunasekaran@ti.com


Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>