dt-bindings: pci: apple,pcie: Drop max-link-speed from example

We no longer use these since 111659c2 (and they never worked
anyway); drop them from the example to avoid confusion.

Fixes: 111659c2 ("arm64: dts: apple: t8103: Remove PCIe max-link-speed properties")
Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220502091308.28233-1-marcan@marcan.st