ARM: shmobile: kzm9g-reference dts: Replace status value "ok" by "okay"

While the DT parsing code recognizes "ok" as a valid value for the
"status" property, ePAPR v1.1 doesn't mention it in the list of valid
values.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>