net: phy: Add SDPX tag based on COPYING file

Some of the PHY and MDIO drivers refer to the COPYING file in the main
directory of this archive. This is the main license for Linux, thus
GPLv2 plus syscall extension.

Fixup the MODULE_LICENSE() where needed and add an SDPX header for
GPLv2.

Cc: David Daney <david.daney@cavium.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>