net: phy: Replace phy driver features u32 with link_mode bitmap

This is one step in allowing phylib to make use of link_mode bitmaps,
instead of u32 for supported and advertised features. Convert the phy
drivers to use bitmaps to indicates the features they support.

Build bitmap equivalents of the u32 values at runtime, and have the
drivers point to the appropriate bitmap. These bitmaps are shared, and
we don't want a driver to modify them. So mark them __ro_after_init.

Within phylib, the features bitmap is currently turned back into a
u32. This will be removed once the whole of phylib, and the drivers
are converted to use bitmaps.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>