PCI: Add pcie_bandwidth_available() to compute bandwidth available to device

Add pcie_bandwidth_available() to compute the bandwidth available to a
device.  This may be limited by the device itself or by a slower upstream
link leading to the device.

The available bandwidth at each link along the path is computed as:

  link_width * link_speed * (1 - encoding_overhead)

2.5 and 5.0 GT/s links use 8b/10b encoding, which reduces the raw bandwidth
available by 20%; 8.0 GT/s and faster links use 128b/130b encoding, which
reduces it by about 1.5%.

The result is in Mb/s, i.e., megabits/second, of raw bandwidth.

Also return the device with the slowest link and the speed and width of
that link.

Signed-off-by: Tal Gilboa <talgi@mellanox.com>
[bhelgaas: changelog, leave pcie_get_minimum_link() alone for now, return
bw directly, use pci_upstream_bridge(), check "next_bw <= bw" to find
uppermost limiting device, return speed/width of the limiting device]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>