dts: renesas_ra: Change to describe the division ratio in a numeric

Move the process of replacing numerical values with macros to
the header, and set the division ratio in a numeric without
using macros in the device tree.

Change `clk-div` defined in `renesas,ra-cgc-pclk.yaml` to `div`.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>