arm64: dts: Drop linux,pci-probe-only from the Seattle DTS

The linux,pci-probe-only property mandates an argument to indicate whether
or not to engage the "probe-only" mode, but the Seattle DTS just provides a
naked property, which is illegal.

Also, it turns out that the board is perfectly happy without probe-only, so
let's drop this altogether.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>