phy: core: add debugfs files

Add a debugfs root for phy class, and create a debugfs directory under
the root when create phy, then phy drivers can add debugfs files.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20230513092218.21139-1-chunfeng.yun@mediatek.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>