stm32f7: rcc: intial stubs support

Reviewed-by: Karl Palsson <karlp@tweak.net.au>
(Added defines and used them)

include/libopencm3/stm32/f7/rcc.h

@@ -159,6 +159,7 @@
 
 /* SWS: System clock switch status */
 #define RCC_CFGR_SWS_SHIFT			2
+#define RCC_CFGR_SWS_MASK			0x3
 #define RCC_CFGR_SWS_HSI			0x0
 #define RCC_CFGR_SWS_HSE			0x1
 #define RCC_CFGR_SWS_PLL			0x2

lib/stm32/f7/Makefile

@@ -40,7 +40,9 @@ TGT_CFLAGS	= -Os -g \
 
 ARFLAGS		= rcs
 
-OBJS		= gpio.o gpio_common_all.o gpio_common_f0234.o
+OBJS		= rcc.o gpio.o gpio_common_all.o gpio_common_f0234.o
+
+OBJS		+= rcc_common_all.o
 
 OBJS		+= rng_common_v1.o
 

lib/stm32/f7/rcc.c

@@ -0,0 +1,11 @@
+#include <libopencm3/stm32/rcc.h>
+
+uint32_t rcc_ahb_frequency = 16000000;
+uint32_t rcc_apb1_frequency = 16000000;
+uint32_t rcc_apb2_frequency = 16000000;
+
+uint32_t rcc_system_clock_source(void)
+{
+	/* Return the clock source which is used as system clock. */
+	return (RCC_CFGR >> RCC_CFGR_SWS_SHIFT) & RCC_CFGR_SWS_MASK;
+}