clk: bcm: rpi: Rename is_prepared function

clk: bcm: rpi: Rename is_prepared function

The raspberrypi_fw_pll_is_on function doesn't only apply to PLL
registered in the driver, but any clock exposed by the firmware.

Since we also implement the is_prepared hook, make the function
consistent with the other function names.

Cc: Michael Turquette <mturquette@baylibre.com>
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/ac93cc4e245316bb7e7426ac5ab0de8f3d919731.1592210452.git-series.maxime@cerno.tech
Signed-off-by: Stephen Boyd <sboyd@kernel.org>