drm: bridge: dw-hdmi: add debugfs node

drm: bridge: dw-hdmi: add debugfs node

Create two debugfs node to debug hdmi controller and phy.

Use following command to debug:
Read hdmi controller register:
	cat /d/dw-hdmi/ctrl
Read hdmi phy register:
	cat /d/dw-hdmi/phy
Write hdmi controller register:
	echo <reg> <val> > /d/dw-hdmi/ctrl
Write hdmi phy register:
	echo <reg> <val> > /d/dw-hdmi/phy

<reg> and <val> is hexadecimal.

Change-Id: I02e40cc94aa651ff0734feddbfa7d816edcf222f
Signed-off-by: Zheng Yang <zhengyang@rock-chips.com>