drm: bridge: dw-hdmi: Define and use macros for PHY register addresses

Replace the hardcoded register address numerical values with macros to
clarify the code.

This change has been tested by comparing the assembly code before and
after the change.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170117082910.27023-18-laurent.pinchart+renesas@ideasonboard.com