arm64: dts: mt8186: Fix CPU map for single-cluster SoC

MT8186 features the ARM DynamIQ technology and combines both two
Cortex-A76 (big) and six Cortex-A55 (LITTLE) CPUs in one cluster:
fix the CPU map to reflect that.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Fixes: 2e78620b ("arm64: dts: Add MediaTek MT8186 dts and evaluation board and Makefile")
Link: https://lore.kernel.org/r/20230126103526.417039-4-angelogioacchino.delregno@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>