pwm: meson: Add clock source configuration for Meson-AXG

For PWM controller in the Meson-AXG SoC, the EE domain and AO domain
have different clock sources. This patch tries to describe them in the
DT compatible data.

Signed-off-by: Jian Hu <jian.hu@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>