boards: bbc_microbit: Update dts and add edge connector node

Align the board dts with the recent changes in the "nordic,nrf-sw-pwm"
binding (remove the no longer existing `channel-count` property) and
add a node representing the edge connector for convenient referring
to SoC pins connected to it.

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