arm64: dts: renesas: r8a774c0: Clean up CPU compatibles

Apparently this DTS crossed over with commit 31af04cd ("arm64: dts:
Remove inconsistent use of 'arm,armv8' compatible string") and missed
out on the cleanup, so put it right.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>