After rebase, the build target NRF52840_DK is using TARGET_NRF5x,
instead of TARGET_NRF5. Moved Cryptocell TRNG related code from
`targets/TARGET_NORDIC/TARGET_NRF5/` to `targets/TARGET_NORDIC/TARGET_NRF5x/`
The unified NRF51 target and feature BLE directories have been
reorganized to follow the naming and directory structure of the
NRF52 implementation.
This reorganization does not include TARGET_MCU_NRF51822 and
derived targets.