drivers: samsung: Introduce exynos-speedy

Speedy is a serial communication bus that is typically used in
Exynos SoC devices for communicating with a PMIC. Implement
basic support for it, as well as add its first user - s2mps17 in
board-dreamlte.

Also, while at it, implement a simple readl function.

Signed-off-by: Ivaylo Ivanov <ivo.ivanov.ivanov1@gmail.com>

lib/unic/string.c

@@ -216,3 +216,8 @@ void writel(unsigned int value, void* address)
 	volatile unsigned int* ptr = (volatile unsigned int*)address;
 	*ptr = value;
 }
+
+uint32_t readl(volatile uint32_t *addr)
+{
+	return *addr;
+}