pwm: meson: Add clock source configuration for Meson G12A

For the PWM controller in the Meson G12A SoC, the EE domain and AO domain
have different clock sources. This patch tries to describe them in the
DT compatible data. The two AO PWM controller has different clock source,
but the first AO controller (A & B) can reuse the AXG parents name.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>