net: phy: move the mscc driver to its own directory

The MSCC PHY driver is growing, with lots of space consuming features
(firmware support, full initialization, MACsec...). It's becoming hard
to read and navigate in its source code. This patch moves the MSCC
driver to its own directory, without modifying anything, as a
preparation for splitting up its features into dedicated files.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>