ARM: dts: dra7xx: replace status value "ok" by "okay"

While the DT parser recognizes "ok" as a valid value for the
"status" property, it is actually mentioned nowhere. Use the
proper value "okay" instead, as done in the majority of files
already.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>