phy: generate swphy registers on the fly

Generate software phy registers as and when requested, rather than
duplicating the state in fixed_phy.  This allows us to eliminate
the duplicate storage of of the same data, which is only different
in format.

As fixed_phy_update_regs() no longer updates register state, rename
it to fixed_phy_update().

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>