arm64: dts: r8a7796: salvator-x: Enable NFS root

Cfr. commit b2407c56 ("arm64: dts: r8a7795: enable nfs root on
Salvator-X board").

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