clk: bcm: rpi: Make the PLLB registration function return a clk_hw

clk: bcm: rpi: Make the PLLB registration function return a clk_hw

The raspberrypi_register_pllb has been returning an integer so far to
notify whether the functions has exited successfully or not.

However, the OF provider functions in the clock framework require access to
the clk_hw structure so that we can expose those clocks to device tree
consumers.

Since we'll want that for the future clocks, let's return a clk_hw pointer
instead of the return code.

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