clk: bcm: rpi: Discover the firmware clocks

The RaspberryPi4 firmware actually exposes more clocks than are currently
handled by the driver and we will need to change some of them directly
based on the pixel rate for the display related clocks, or the load for the
GPU.

Since the firmware implements DVFS, this rate change can have a number of
side-effects, including adjusting the various PLL voltages or the PLL
parents. The firmware also implements thermal throttling, so even some
thermal pressure can change those parameters behind Linux back.

DVFS is currently implemented on the arm, core, h264, v3d, isp and hevc
clocks, so updating any of them using the MMIO driver (and thus behind the
firmware's back) can lead to troubles, the arm clock obviously being the
most problematic.

In order to make Linux play as nice as possible with those constraints, it
makes sense to rely on the firmware clocks as much as possible. However,
the firmware doesn't seem to provide some equivalents to their MMIO
counterparts, so we can't really replace that driver entirely.

Fortunately, the firmware has an interface to discover the clocks it
exposes.

Let's use it to discover, register the clocks in the clocks framework and
then expose them through the device tree for consumers to use them.

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: linux-clk@vger.kernel.org
Acked-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Tested-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/438d73962741a8c5f7c689319b7443b930a87fde.1592210452.git-series.maxime@cerno.tech


Signed-off-by: Stephen Boyd <sboyd@kernel.org>