pci: pcie_dw_rockchip: Support max_link_speed dts property

Add support for max_link_speed specified in the PCI DT binding.

Signed-off-by: Jon Lin <jon.lin@rock-chips.com>
[eugen.hristev@collabora.com: port to latest API, set default correctly,
align to 80 chars]
Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
[jonas@kwiboo.se: switch to dev_read_u32_default]
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>