hw/mcu/stm32: Add option to set HSE_VALUE from syscfg

[kasjer]

This adds STM32_CLOCK_HSE_VALUE that can be used to specify HSE frequency in mynewt style. It could be always set w macro HSE_VALUE but this makes it easier to manipulate this value in similar way as most other parameters.

Signed-off-by: Jerzy Kasenberg [email protected]