clk: bcm: rpi: Allow the driver to be probed by DT

clk: bcm: rpi: Allow the driver to be probed by DT

The current firmware clock driver for the RaspberryPi can only be probed by
manually registering an associated platform_device.

While this works fine for cpufreq where the device gets attached a clkdev
lookup, it would be tedious to maintain a table of all the devices using
one of the clocks exposed by the firmware.

Since the DT on the other hand is the perfect place to store those
associations, make the firmware clocks driver probe-able through the device
tree so that we can represent it as a node.

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: linux-clk@vger.kernel.org
Signed-off-by: Maxime Ripard <maxime@cerno.tech>