PCI: brcmstb: support BCM4908 with external PERST# signal controller

BCM4908 uses external MISC block for controlling PERST# signal. Use it
as a reset controller.

Link: https://lore.kernel.org/r/20201210180421.7230-3-zajec5@gmail.com


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