arm64: exynos: Enable Exynos Multi-Core Timer driver

Some ARM64 Exynos SoCs have MCT timer block, e.g. Exynos850 and
Exynos5433. CLKSRC_EXYNOS_MCT option is not visible unless COMPILE_TEST
is enabled. Select CLKSRC_EXYNOS_MCT option for ARM64 ARCH_EXYNOS like
it's done in arch/arm/mach-exynos/Kconfig, to enable MCT timer support
for ARM64 Exynos SoCs.

Even though ARM architected timer is available on all ARM64 SoCs, and
used for managing timer hardware and clock source events, MCT timer
still can be used as a wakeup source, as stated in commitae460fd9164b
("clocksource/drivers/exynos_mct: Prioritise Arm arch timer on arm64").
It's also nice to be able to test available MCT IP-core.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Link: https://lore.kernel.org/r/20211101193531.15078-3-semen.protsenko@linaro.org
Link: https://lore.kernel.org/r/20211220165004.17005-2-krzysztof.kozlowski@canonical.com

'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>