drivers: i2c: nrfx_twim_rtio: use msg_buf for rx if required

The DMA buffer needs to be used both for tx and rx if required.
Extend RTIO driver variant to use DMA buffer and copy received
data from it to user buffer.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>