Support for nRF52 without soft devices

[edwardalee]

The support for bare-metal code generation for nRF52 currently only works when soft devices are used. This is not unreasonable because the wireless capabilities of the nRF52 are unlikely to work without the soft devices. Nevertheless, for generality, there should be an option in the target's platform attribute to not use a soft device. In particular, the lf_nrf52_support.c uses functions sd_nvic_critical_region_exit and sd_nvic_critical_region_enter that will not be available if a soft device is not used, resulting in a linker error on the generated code. Those functions will need to be replaced with the lower-level ARM function __enable_irq and __disable_irq.