drivers: clock_control: renesas_ra: Adding macros to convert DT values

Adding the macros `RA_CGC_CLK_SRC` and `RA_CGC_CLK_DIV` that derive
the BSP clock settings from the DeviceTree node settings.
I also define some aliases to fill in the gaps with the BSP
naming conventions.

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