phy: core: Add devm_of_phy_get to phy-core

Adding devm_of_phy_get will allow to get phys by supplying a
pointer to the struct device_node instead of struct device.

Signed-off-by: Kamil Debski <k.debski@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>