clk: renesas: rzg2l: Add struct clk_hw_data

Add clk_hw_data struct that keeps the core part of the clock data.
sd_hw_data embeds a member of type struct clk_hw_data along with other
members (in the next commits).  This commit prepares the field for
refactoring the SD MUX clock driver.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20230929053915.1530607-9-claudiu.beznea@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>