clk: bcm2835: Don't cache the PLLB rate

The PLLB rate will be changed through the firmware clocks drivers and will
change behind this drivers' back, so we don't want to cache the rate.

Acked-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Tested-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/9864daba2f584ed49aee5ed1d2f4d48507c58197.1592210452.git-series.maxime@cerno.tech


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