dt-bindings: PCI: brcmstb: add BCM4908 binding

BCM4908 is a SoC family with PCIe controller sharing design with the one
for STB. BCM4908 has different power management and memory controller so
few tweaks are required.

PERST# signal on BCM4908 is handled by an external MISC block so it
needs specifying a reset phandle.

Link: https://lore.kernel.org/r/20201210180421.7230-2-zajec5@gmail.com


Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>