ARM: dts: s3c24xx: fix number of PWM cells

The PWM has only three cells, not four, as pointed out by dtschema:

  timer@51000000: #pwm-cells:0:0: 3 was expected

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200907183313.29234-8-krzk@kernel.org