emul: Only add enabled DT nodes to bus emulators

The eSPI, I2C, and SPI emulators use devicetree macros to build an array
of devices on the virtual bus. Currently, they will add device nodes that
are not status-okay. This leads to linker errors because the respective
device drivers would not have instantiated device structs for these
nodes --assuming the driver was even compiled. This can be frustrating
if nodes need to be disabled for debugging or configuration purposes.

Update the bus emulators to only consider status-okay nodes by changing
the macros used to iterate over bus devices.

Signed-off-by: Tristan Honscheid <honscheid@google.com>