clk: bcm: rpi: Create helper to retrieve private data

The RaspberryPi firmware clocks driver uses in several instances a
container_of to retrieve the struct raspberrypi_clk_data from a pointer
to struct clk_hw. Let's create a small function to avoid duplicating it
all over the place.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>