net: dsa: remove unnecessary phy*.h includes

Including phy.h and phy_fixed.h into net/dsa.h causes phy*.h to be an
unnecessary dependency for quite a large amount of the kernel.  There's
very little which actually requires definitions from phy.h in net/dsa.h
- the include itself only wants the declaration of a couple of
structures and IFNAMSIZ.

Add linux/if.h for IFNAMSIZ, declarations for the structures, phy.h to
mv88e6xxx.h as it needs it for phy_interface_t, and remove both phy.h
and phy_fixed.h from net/dsa.h.

This patch reduces from around 800 files rebuilt to around 40 - even
with ccache, the time difference is noticable.

Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>