ARM: EXYNOS: Simplify code in coupled CPU idle hot path

exynos_enter_aftr() is called by coupled CPU idle code every time CPU
enters idle state, what can be considered as a hot path. Replace
of_machine_is_compatible() call with a simple SoC revision check.

of_machine_is_compatible() function performs a dozen of string comparisons
during the full device tree walk, while soc_is_exynos4412() is a simple
integer check on SoC revision variable.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>