Document ext reset used in ipq8064 SoC by qcom PCIe driver.

Link: https://lore.kernel.org/r/20200615210608.21469-6-ansuelsmth@gmail.comSigned-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Stanimir Varbanov <svarbanov@mm-sol.com>