phy: phy-core: allow specifying supply at port level

Multi-port phys may have per port power supplies. Let's change phy
core to look for supply at the port level when multiple ports are
specified. To keep compatibility with the existing device tree board
descriptions for single-port phys we will continue looking up the
power supply at the parent node level

Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Arun Ramamurthy <arun.ramamurthy@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>