SPI: Various fixes (examples, mem leak, arg check)
- Fix SPI master example to use DMA-capable memory for display initialization. Fixes https://github.com/espressif/esp-idf/issues/551
- SPI master: Do not leak DMA descriptor pointer array on free
- SPI Master/Slave: Check if DMA'ed buffers actually live in DMA-capable memory
See merge request !735