arm64: dts: apple: t600x: Disable unused PCIe ports

The PCIe ports are unused (without devices) so disable them instead of
removing them.

Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Hector Martin <marcan@marcan.st>