drivers: serial: nrfx: Ensure that instances have some pins assigned

Add build assertions that will ensure that every peripheral for
which a driver instance is created has some pins assigned to it.
Neither pinctrl-0 nor *-pin properties can be currently marked as
required in devicetree, so these assertions will help users avoid
invalid configurations where it could be hard to figure out why
the UART is not working.

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