net: dsa: mv88e6xxx: add field to specify internal phys layout

mv88e6xxx currently assumes that switch equipped with internal phys have
those phys mapped contiguously starting from port 0 (see
mv88e6xxx_phy_is_internal). However, some switches have internal PHYs but
NOT starting from port 0. For example 88e6393X, 88E6193X and 88E6191X have
integrated PHYs available on ports 1 to 8
To properly support this offset, add a new field to allow specifying an
internal PHYs layout. If field is not set, default layout is assumed (start
at port 0)

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>