cpu/stm32: added support for STM32F7

cpu/stm32_common/cpu_common.c

cpu/stm32_common/include/periph_cpu_common.h

cpu/stm32_common/include/stmclk.h

cpu/stm32_common/periph/uart.c

cpu/stm32f7/Makefile

cpu/stm32f7/Makefile.include

cpu/stm32f7/cpu.c

cpu/stm32f7/include/cpu_conf.h

cpu/stm32f7/include/periph_cpu.h

cpu/stm32f7/include/vendor/stm32f746xx.h

cpu/stm32f7/ldscripts/stm32f746zg.ld

cpu/stm32f7/periph/Makefile

cpu/stm32f7/stmclk.c

cpu/stm32f7/vectors.c