boards: arduino_nano_33_ble: Correct assignment of P0.13 to PWM

This is a follow-up to commit f4a0ddd8.

Since the yellow LED and the SCK line of spi2 use the same pin
(P0.13), they cannot be used together. Consequently, the pin
should not be assigned to the same PWM instance as other pins
that drive LEDs, as the limitation of usage would apply to the
whole PWM instance (it acquires all the pins assigned to it on
initialization of the PWM driver, regardless of whether the PWM
signal is eventually generated on particular outputs or not).

Use a separate PWM instance (disabled by default) for driving
the yellow LED. Don't use the "nordic,invert" property for that
PWM, as the yellow LED is active high.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>