ARM: dts: meson8: add the PWM controller nodes

pwm_ab and pwm_cd are already inherited from meson.dtsi, we only need to
define the correct "compatible" string so the pwm-meson driver can
choose the parent clocks correctly.
pwm_ef is added to meson8.dtsi directly (similar to how it's done in
meson8b.dtsi) as this controller only exists on Meson8 and Meson8b.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>