net: mvmdio: enhance driver to support SMI error/done interrupts

This patch enhances the "mvmdio" to support a SMI error/done interrupt
line which can be used along with a wait queue instead of doing
busy-waiting on the registers. This is a feature which is available in
the mv643xx_eth SMI code and thus reduces again the gap between the two.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>