spi: Mark instantiated device nodes with OF_POPULATE

Mark (and unmark) device nodes with the POPULATE flag as appropriate.
This is required to avoid multi probing when enabling and populating SPI
buses in DT overlays.

Based on commit 4f001fd3 ("i2c: Mark instantiated device nodes
with OF_POPULATE").

Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>