phy: move fixed_phy MII register generation to a library

Move the fixed_phy MII register generation to a library to allow other
software phy implementations to use this code.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>