boards: arm: nucleo_f091rc: define PWM LEDs

The board had PWM LEDs defined in the blinky_pwm sample, mainly because
it conflicts with SPI1 pinmux. A better approach is to still define the
PWM LEDs in the board but keep the PWM controller disabled by default.
Then, samples just need to enable the PWM controller (and disable SPI if
used simultaneously).

Also updated period to 20ms (reasonable value for a PWM-driven LED).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>