Make struct meson_pwm_channel accessible from struct meson_pwm.

PWM core has a limitation: per-channel data can only be set after
pwmchip_add() is called. However, pwmchip_add() internally calls
pwm_ops.get_state(). If pwm_ops.get_state() needs access to the
per-channel data it has to obtain it from struct pwm_chip and struct
pwm_device's hwpwm information.

Add a struct meson_pwm_channel for each PWM channel to struct meson_pwm
so the pwm_ops.get_state() callback can be implemented as it needs
access to the clock from struct meson_pwm_channel.
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>