ARM: dts: at91: sama5d4_xplained: Remove spi0 node

sama5d4_xplained has an optional on-board serial DataFlash (AT25DF321A),
which does not come populated on board by default. Since the spi0 node
does not have any child populated by default, thus no user on
sama5d4_xplained, remove it. This avoids the following error in SPI NOR,
which reads the values of the pulled downed lines, unrecognizing the flash:

spi-nor spi0.0: unrecognized JEDEC id bytes: 00 00 00 00 00 00

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210716154739.835738-1-tudor.ambarus@microchip.com