net/ftgmac100: Add NC-SI mode support

Update the ftgmac100 driver to support NC-SI instead of an mdio phy
where available. This is a common setup for Aspeed AST2x00 platforms.

NC-SI mode is determined from the device-tree if either phy-mode sets it
or the use-ncsi property exists. If set then normal mdio setup is
skipped in favour of the NC-SI phy.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>