RCC导出函数

模块
	HSE
	HSI
	HSI48
	LSE
	LSI
	LSCO
	System
	MCO
	Peripheral Clock Source
	RTC
	PLL
	FLAG Management
	IT Management
	De-initialization function
	Get system and peripherals clocks frequency functions
	返回芯片上不同时钟的频率；系统时钟、AHB、APB1和APB2总线时钟以及设备上可用的不同外设时钟。