dt-bindings: PCI: brcmstb: New prop 'aspm-no-l0s'

For various reasons, one may want to disable the ASPM L0s
capability.

Link: https://lore.kernel.org/r/20200507201544.43432-4-james.quinlan@broadcom.com
Signed-off-by: Jim Quinlan <jquinlan@broadcom.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>