clocksource: exynos_mct: Add the support for ARM64

clocksource: exynos_mct: Add the support for ARM64

This patch allows building and compile-testing the driver also for
ARM64.  The delay_timer is only supported on ARMv7.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Kukjin Kim <kgene@kernel.org>
Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
[k.kozlowski: Adjusted commit msg]
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>