ARM: EXYNOS: Switch to syscon reboot and poweroff

Since we switch to use generic syscon regmap based reset/poweroff
driver for exynos SoC, lets select it from ARCH_EXYNOS instead of
enabling it from various defconfigs. This also select POWER_RESET
as SYSCON-{reset, poweroff} drivers depends on it.

Also because of the above changes we don't need special handling
for reboot/poweroff in exynos pmu driver. This patch also removes
the same from exynos pmu driver to keep git bisect working.

Note: This will break reboot/poweroff on boards with older dtbs
with a newer kernel.

Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>