clk: composite: rename 'div' references to 'rate'

Rename all div_hw and div_ops related variables and functions to use
rate_hw, rate_ops, etc.  This is to make the rate-change portion of the
composite clk implementation more generic.  A patch following this one
will allow for fixed-rate clocks to reuse this infrastructure.

Signed-off-by: Mike Turquette <mturquette@linaro.org>
Reviewed-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Tested-by: Emilio López <emilio@elopez.com.ar>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>