drm/vc4: hdmi: Add PHY RNG enable / disable function

Let's continue the implementation of hooks for the parts that change in the
BCM2711 SoC with the PHY RNG setup.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/216155f1c0b83e622ac60a2f7d00eb707de3acba.1599120059.git-series.maxime@cerno.tech