arm64: defconfig: enable syscon-poweroff driver

Enable the generic syscon-poweroff driver used on all Exynos ARM64 SoCs
(e.g. Exynos5433) and few APM SoCs.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Link: https://lore.kernel.org/r/20230901115732.45854-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>