soc: samsung: exynos-pmu: Fix compilation when nothing selects CONFIG_MFD_CORE

Commit 93618e34 ("soc: samsung: exynos-pmu: instantiate clkout
driver as MFD") adds a "devm_mfd_add_devices" call in the exynos-pmu
driver which depends on CONFIG_MFD_CORE. If no driver selects that
config, the build will fail if CONFIG_EXYNOS_PMU is enabled with the
following error:

  drivers/soc/samsung/exynos-pmu.c:137: undefined reference to `devm_mfd_add_devices'

Fix this by making CONFIG_EXYNOS_PMU select CONFIG_MFD_CORE.

Fixes: 93618e34 ("soc: samsung: exynos-pmu: instantiate clkout driver as MFD")
Cc: <stable@vger.kernel.org>
Signed-off-by: David Virag <virag.david003@gmail.com>
Link: https://lore.kernel.org/r/20210909222812.108614-1-virag.david003@gmail.comSigned-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>

drivers/soc/samsung/Kconfig

@@ -25,6 +25,7 @@ config EXYNOS_PMU
 	bool "Exynos PMU controller driver" if COMPILE_TEST
 	depends on ARCH_EXYNOS || ((ARM || ARM64) && COMPILE_TEST)
 	select EXYNOS_PMU_ARM_DRIVERS if ARM && ARCH_EXYNOS
+	select MFD_CORE
 
 # There is no need to enable these drivers for ARMv8
 config EXYNOS_PMU_ARM_DRIVERS