boards: rpi_pico: avoid conflicting use of LED

led0 and pwm_led0 are both referring to the same LED. The usage is
mutualy exclusive, as only one pin muxing can be applied.

Disabling child nodes is currently neither supported by gpio-leds device
nor by pwm-leds device. We need to disable either of the devices
completely for correct operation.

Please note that even if disabling of child nodes would be supported, the
devices would be empty anyway, as both have only one singe LED defined at
the moment. Therefore we can completely disable one of the devices in any
case.

Fixes: b876ad9d ("boards: arm: rpi_pico: add pwm bindings to
  devictree")
Signed-off-by: Oliver Barta <o.barta89@gmail.com>