dt-bindings: reset: document Broadcom's BCM4908 PCIe reset binding

BCM4908 was built using older PCIe hardware block that requires using
external reset block controlling PERST# signals.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>