boards: Add alias to boards with spi-flash node

This is continuation of the 64fbbd9a
where spi-flash0 alias was defined for some of boards' devicetrees.
It's needed to turn on building samples/drivers/spi_flash for boards
with "nordic,qspi-nor" compatible.

Signed-off-by: Adam Wojasinski <adam.wojasinski@nordicsemi.no>