tests: drivers: build_all: Remove dts_fixup.h replace with overlay

Remove the dts_fixup.h with a DTS app overlay file instead.  We have
sensors that show up on both the I2C and SPI busses, and we disable
the sensors to match how the tests work currently.

The app.overlay utilizes the testing dts bindings to create test nodes
that should not conflict with any existing SoC nodes.  Thus allowing the
test to run on any platform.

The dts overlay is structured such that i2c.dtsi and spi.dtsi are
generated with default values for required properties.  We than update
device properties for those nodes that the defaults are sufficient for
one reason or another.

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