arm64: dts: renesas: r8a77970: Remove non-existing STBE region

R-Car V3M does not have the Stream Buffer for EtherAVB-IF (STBE).

Note that the RAVB driver does not use this region.

Fixes: bea2ab13 ("arm64: dts: renesas: r8a77970: add EtherAVB support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>