mtd: spi-nor: always use bounce buffer for register read/writes

spi-mem layer expects all buffers passed to it to be DMA'able. But
spi-nor layer mostly allocates buffers on stack for reading/writing to
registers and therefore are not DMA'able. Introduce bounce buffer to be
used to read/write to registers. This ensures that buffer passed to
spi-mem layer during register read/writes is DMA'able. With this change
nor->cmd-buf is no longer used, so drop it.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>