Adding Nucleo_f401re with examples

[FredPraca]

Adding files for Nucleo_f401re in order to fix #313

[CLAassistant]

All committers have signed the CLA.

[Fabien-Chouteau]

Thanks @FredPraca,

Where are the arch/ARM/STM32/devices/stm32f401/stm32-adc.ad*, arch/ARM/STM32/devices/stm32f401/stm32-timers.ad* and other drivers coming from?
It looks like they are copied from arch/ARM/STM32/drivers.

[FredPraca]

For stm32-adc, STM32F401 doesn't support multi-mode.
For timers, TIM6 and TIM7 don't exist and there are no basic timers for STM32F401 or STM32F411.
For PWM, the fact that several timers are missing in these series has an impact on has_APB1_Frequency.

[FredPraca]

After reading #340, it seems that redefining these files was a bad idea but that's the only one I got to make it work for both Nucleo F401 and Nucleo F411 in a simple way.