drivers: i2c: nrfx_twim_rtio: fix dma buf section attr

The buffer used for the TWIM DMA needs to be allocated to the
section provided with the memory-regions dt prop. The macros for
doing this where malformed. This commit fixes and cleans up the
macros.

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