Add support for the led-mode property for the following PHYs
which have a single LED mode configuration value.

KSZ8001 and KSZ8041 which both use register 0x1e bits 15,14 and
KSZ8021, KSZ8031 and KSZ8051 which use register 0x1f bits 5,4
to control the LED configuration.
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>