spi: shell: add simple SPI shell

Inspired by I2C shell. Useful during
SPI device driver development or for debugging.

Usage example (read JEDEC ID):

```
uart:~$ spi conf spi4 1000000 oh
uart:~$ spi transceive 9f 00 00 00
TX:
00000000: 9f 00 00 00                                 |....             |
RX:
00000000: 00 ef 40 19                                 |....             |
```

Signed-off-by: Nerijus Bendžiūnas <nerijus.bendziunas@astrolightspace.com>