switchtec: Improve MRPC efficiency by enabling write combining

The MRPC Input buffer is mostly memory without any side effects, so we
can improve the access time by enabling write combining on this region
of the BAR.

In a few places, we still need to flush the WC buffer.  To do this, we
simply read from the Outbound Doorbell register because reads to this
register are processed by low latency hardware.

Signed-off-by: Kelvin Cao <kelvin.cao@microchip.com>
Signed-off-by: Wesley Sheng <wesley.sheng@microchip.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>