ARM: dts: kzm9d: Add rw parameter to bootargs

Add rw as bootargs parameter to make the KZM9D board bootargs match other
boards from Renesas. No need to be special.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>