drivers: spi: spi_nrfx_spim: Add support for RX buffer from RAM region

This patch adds support for RX buffer placed by a linker in
memory region defined in SPIM devicetree node. The buffer is placed
in memory region defined as devicetree node. The memory region node's
reference is then stored in `memory-regions` property of SPIM node.

Added build time assertion to check if `CONFIG_SPI_NRFX_RAM_BUFFER_SIZE`
Kconfig symbol has value greater than 0 when given SPIM node has
`memory-region` property.

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