clk: agilex: Add clock driver for Agilex

Add clock manager driver for Agilex. Provides clock initialization
and get_rate functions.

agilex-clock.h is from Linux commit ID cd2e1ad12247.

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>