net: dsa: mv88e6xxx: factorize switch reset

Add a MV88E6XXX_FLAG_PPU_ACTIVE flag to describe how to reset the
switch, and merge the reset call to the common setup code.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>