net: phy: add phy_interface_t bitmap support

Add support for a bitmap for phy interface modes, which includes:
- a macro to declare the interface bitmap
- an inline helper to zero the interface bitmap
- an inline helper to detect an empty interface bitmap
- inline helpers to do a bitwise AND and OR operations on two interface
  bitmaps

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>