ARM64: dts: meson-axg: add PWM DT info for Meson-Axg SoC

Add PWM DT info for the Amlogic's Meson-Axg SoC.

Signed-off-by: Jian Hu <jian.hu@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>