[DRIVER] Make reboot/halt-delay configurable in device-tree
- a reboot/halt-delay can be configured in the range 0-2550ms by the device-tree-property "reboot-delay-ms"
- the defines for the STM32-registers are replaced by an enum, this is easier to extend
- the function stm32_request is renamed to stm32_gpio_request to better match the naming convention