dts: surpress node name for SPI buses should be 'spi' warning

On Atmel & Silabs SoCs the SPI controller is implemented on a shared
peripheral block (sercom for atmel, usart on silabs) so we can't have
the node name be "spi@...".  In these cases we disable the warning
via passing '-Wno-spi_bus_bridge' to dtc.

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