clk: keystone: sci-clk: cache results of last query rate operation

clk: keystone: sci-clk: cache results of last query rate operation

Cache results of the latest query rate operation. This optimizes the
firmware interface a bit, avoiding unnecessary calls to firmware if we
know the result already; the firmware interface is pretty expensive
to use for query rate functionality.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Link: https://lore.kernel.org/r/20200907085740.1083-3-t-kristo@ti.com
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>