dts: bindings: nordic,nrf-qdec: Remove requirement for a-pin and b-bin

This is a follow-up to commit 1a01ca2a.

Since support for pinctrl has been added to the qdec_nrfx driver,
the related binding can no longer require the `a-pin` and `b-pin`
properties to be defined.

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