doc: board: phytec: phycore-am6: Use mtd commands

Update Flash to SPI NOR chapter for use with mtd commands.
This is more convenient as we do not have to remember any
offsets in the SPI.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Reviewed-by: Daniel Schultz <d.schultz@phytec.de>
2 files changed