add support USART1 & USART6 in usart_baud(), because USART1 & USART6 clock source is APB2/PCLK2, others USARTx is APB1/PCLK1

current the kernel setting STM32F4xx CPU Clock/AHB (HCLK) run in 168MHz
PCLK1: 42MHz, PCLK2: 84MHz

reference: Table 2. STM32F4xx register boundary RM0090 Reference manual
(DM00031020.pdf, http://www.st.com/web/en/resource/technical/document/reference_manual/DM00031020.pdf)
add DBGPORT_USE_USART1 options
Should pick up it for STM32F429I discovery board
Merge pull request #69 from KunYi/master

platform: add USART1 configurations for STM32F4x