dts/nxp: Fix dtc warning with spi device node name

Fix the following dtc warning:

mimxrt1064_evk.dts.pre.tmp:78.31-85.5: Warning (spi_bus_bridge):
 /soc/flexspi1@402a4000: node name for SPI buses should be 'spi'

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>