Generic bitbanged MDIO library

Previously, bitbanged MDIO was only supported in individual
hardware-specific drivers.  This code factors out the higher level
protocol implementation, reducing the hardware-specific portion to
functions setting direction, data, and clock.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>