clocksource/drivers/exynos_mct: Remove dead code

Exynos Multi-Core Timer driver is used only on device-tree based
systems, so remove non-dt related code. In case of !CONFIG_OF
the code is anyway equal because of_irq_count() has a stub
returning 0. Device node pointer is always provided when driver
has been probed from device tree.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>