ARM: dts: s3c24xx: align PWM/timer node name with dtschema

Although PWM is used on S3C24xx as clocksource/timer, the dtschema
expects the node to be named in certain format:

  timer@51000000: $nodename:0: 'timer@51000000' does not match '^pwm(@.*|-[0-9a-f])*$'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200907183313.29234-10-krzk@kernel.org