edtlib: handle match of fixed-partition on any bus

If we have a fixed-partition on a flash device that is for example on
a spi controller we will not get a binding match currently.  This is
because we expect a match between both the compatible and the fact that
fixed-partition node is a decendant of the spi bus.

To address this we treat fixed-partitions as if they are on no bus.
This has the effect of causing a binding match as well as ensuring that
when we process the fixed-partition node we will do anything special to
it because of the bus it happens to be under (for example SPI CS_GPIO
processing).

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