pwm: meson: Add missing spin_lock_init()

The driver uses the spin_lock but does not initialize it. Fix it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>