mv643xx_eth: add tx rate control

Add an interface for the hardware's per-port and per-subqueue
TX rate control.  In this stage, this is mainly so that we can
disable the bandwidth limits during initialisation of the port.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>