drm: imx: imx-hdmi: split phy configuration to platform driver

hdmi phy configuration is platform specific, which can be adjusted
according to the board to get the best SI

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>