ARM: EXYNOS: Fix UART address selection for DEBUG_LL

The Exynos542x SoCs using A15+A7 can boot to A15 or A7. If it boots using
A7 (like on Odroid XU family boards), it can't choose right UART
physical address only the part number of CP15. Fix the detection logic
by checking the Cluster ID additionally.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
[k.kozlowski: Extend commit message]
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>