dt: bcm283x: Change BCM283x HDMI to use firmware clock driver

The clk-bcm2835 handling of the pixel clock does not function
correctly when the HDMI power domain is disabled.

The firmware supports it correctly, and the firmware clock
driver now supports it, so switch the vc4-hdmi driver to use
the firmware clock driver.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>