net: ftgmac100: add MDIO bus and phylib support

Implement the MDIO bus read/write functions using the readl_poll_timeout()
routine, initialize the bus and scan for the PHY. RGMII and RMII mode
are supported.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>