rockchip: clk: rk3399: remove unused fields from priv-structures

This removes the unused 'rate' field from both rk3399_pmuclk_priv and
rk3399_clk_priv. I didn't bother to check where this came from (i.e.
what the historical context of these was), but only verified that
these are indeed unused across all code-paths.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>