arm64: dts: renesas: r8a77970: Update IPMMU DS1 bit number

Judging by "R-Car-Gen3-rev0.80" IPMMU IMSSTR register documentation
for [R-Car V3M] the DS1 bit field should be bit 0.

Update the ipmmu-main property to make it match the data sheet.

Fixes: ce3b52a1 ("arm64: dts: renesas: r8a77970: Add IPMMU device nodes")
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>