drivers: serial: uart_nrfx_uarte: utilize EasyDMA property from dts

This commit aligns UARTE shim to utilize memory-region property from
nordic,nrf-uarte compatible. 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>