soc: samsung: Add Exynos PMU driver

Add basic Power Management Unit (PMU) driver for Exynos SoCs. For now
it's only capable of changing UART path in PMU, which is needed for
E850-96 board. The driver's structure resembles the exynos-pmu driver
from Linux kernel, and although it's very basic and slim at the moment,
it can be easily extended in future if the need arises.

UCLASS_NOP is used, as there are no benefits in using more elaborate
classes like UCLASS_MISC in this case. The DM_FLAG_PROBE_AFTER_BIND flag
is added in bind function, as the probe function must be always called
for this driver.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
diff --git a/drivers/soc/samsung/Kconfig b/drivers/soc/samsung/Kconfig
index ffb87fe..737b7ca 100644
--- a/drivers/soc/samsung/Kconfig
+++ b/drivers/soc/samsung/Kconfig
@@ -5,6 +5,16 @@
 
 if SOC_SAMSUNG
 
+config EXYNOS_PMU
+	bool "Exynos PMU controller driver"
+	depends on ARCH_EXYNOS
+	select REGMAP
+	select SYSCON
+	help
+	  Enable support for system controller configuration driver. It allows
+	  one to configure system controller registers (e.g. some register in
+	  PMU syscon) by providing register's offset, mask and value.
+
 config EXYNOS_USI
 	bool "Exynos USI (Universal Serial Interface) driver"
 	depends on ARCH_EXYNOS