ARM: dts: Make CPU configuration more readable on exynos542x/5800

Exynos5420 and Exynos5800 boards boot from big core (A15) but
Exynos5422 boards choose otherwise: LITTLE core (A7) (on Exynos5422 this
is property of the board - configurable by pulling up/down gpg2-1).
To make user-visible CPU ordering more consistent the 'cpus' node was
overridden by exynos5422-cpus.dtsi.

However this is a little bit ugly and error-prone. Overriding the CPU
child nodes requires to basically reverse what was done initially in
exynos5420.dtsi.

Instead, split CPU configuration entirely to separate files which should
be included by board DTS.

Suggested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Chanho Park <parkch98@gmail.com>