soc: nrf53: Fix extraction of the XOSC32MTRIM.SLOPE bitfield

The value in this bitfield is provided in the two's complement form,
so it requires special handling. Previously, it was read as just an
unsigned value and this could result in a wrongly computed CAPVALUE.

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