board: teensy: drop spi-nor from the flash nodes

These are RT1060 based boards that use the flash for XIP, the bus
(nxp,imx-flexspi) lives under memc or flash. Declaring the flash as
"jedec,spi-nor" causes the SPI NOR driver to create an instance for it,
but then there's no bus and the build fails.

Dropping the spi-nor compatible seems to fix the problem.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>