drivers: i2c: i2c_nrfx_twim: Utilize memory-region prop from devicetree

This commit aligns TWIM shim to utilize memory-region property.
The memory-region is not required property that enables user
to specify placement of dma buffers in memory region.
It is done by assigning to memory-region property,
phandle to node with zephyr,memory-region and mimo-sram compatible.

When memory-region property is not specified for given
instance, buffer is placed in default RAM region with other data.

Signed-off-by: Adam Wojasinski <adam.wojasinski@nordicsemi.no>