clk: renesas: r8a7796: Add PWM clock

This patch adds PWM clock for PWM.

Signed-off-by: Ryo Kodama <ryo.kodama.vz@renesas.com>
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
[geert: Correct parent clock]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>