phy: core: Let node ptr of PHY point to PHY and not of PHY provider

In case of multi-phy PHY providers, each PHY should be modeled as a sub
node of the PHY provider. Then each PHY will have a different node pointer
(node pointer of sub node) than that of PHY provider. Added this provision
in the PHY core.
Also fixed all drivers to use the updated API.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Lee Jones <lee.jones@linaro.org>