phy: renesas: convert to SPDX identifiers

This patch updates license to use SPDX-License-Identifier
instead of verbose license text.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>