cmd: pci: Add command to set MPS of all PCIe devices

Enable tuning of the PCI Express MPS (Maximum Payload Size) of
each device. The Maximum Read Request Size is not altered.

The SAFE method uses the largest MPS value supported by all devices in the
system for each device. This method is the same algorithm as used by Linux
pci=pcie_bus_safe.

The PEER2PEER method sets all devices to the minimal (128 byte) MPS, which
allows hot plug of devices later that might only support the minimum size,
and ensures compatibility of DMA between two devices on the bus.

Signed-off-by: Stephen Carlson <stcarlso@linux.microsoft.com>