devicetree: Adjust status "ok" -> "okay" under drivers/of/

Tweak a small number of status "ok" lines in a single file under
/drivers/of/ to use the proper DTSpec 0.1 spelling of "okay".

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Rob Herring <robh@kernel.org>