[stm32] share power.c across stm32 platforms.

Previously power.c was only implemented for f7, but the
same functionality exists in other stm32 devices, thanks
to CMSIS defines.

This refactors out f7's power.c to platform/stm32/ and adds
deps from f7 and f4.

platform/stm32/rules.mk

@@ -0,0 +1,10 @@
+# Code shared across different stm32 platforms.
+
+LOCAL_DIR := $(GET_LOCAL_DIR)
+
+MODULE := $(LOCAL_DIR)
+
+MODULE_SRCS += \
+	$(LOCAL_DIR)/power.c
+
+include make/module.mk

platform/stm32f4xx/rules.mk

@@ -47,6 +47,7 @@ LINKER_SCRIPT += \
 	$(BUILDDIR)/system-twosegment.ld
 
 MODULE_DEPS += \
+	platform/stm32 \
 	platform/stm32f4xx/STM32F4xx_StdPeriph_Driver \
 	arch/arm/arm-m/systick \
 	lib/cbuf \

platform/stm32f7xx/rules.mk

@@ -43,7 +43,6 @@ MODULE_SRCS += \
 	$(LOCAL_DIR)/flash.c \
 	$(LOCAL_DIR)/gpio.c \
 	$(LOCAL_DIR)/init.c \
-	$(LOCAL_DIR)/power.c \
 	$(LOCAL_DIR)/timer.c \
 	$(LOCAL_DIR)/uart.c \
 	$(LOCAL_DIR)/usbc.c \
@@ -60,6 +59,7 @@ LINKER_SCRIPT += \
 	$(BUILDDIR)/system-twosegment.ld
 
 MODULE_DEPS += \
+	platform/stm32 \
 	platform/stm32f7xx/STM32F7xx_HAL_Driver \
 	arch/arm/arm-m/systick \
 	dev/gpio \