PCI: qcom: Support pci speed set for ipq806x

PCI: qcom: Support pci speed set for ipq806x

Some SoC based on ipq8064/5 needs to be limited to pci GEN1 speed due to
some hardware limitations. Add support for speed setting defined by the
max-link-speed binding. If not defined the max speed is set to GEN2 by
default.

Link: https://lore.kernel.org/r/20200615210608.21469-12-ansuelsmth@gmail.com
Signed-off-by: Sham Muthayyan <smuthayy@codeaurora.org>
Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Stanimir Varbanov <svarbanov@mm-sol.com>