MLBEDSW-3796 Ethos-U driver interface multiple NPUs

---ethosu_driver---
Modified: Declarations for the driver interfaces to support multiNPU (takes *drv)
Added: ethosu_register_driver(...) to allow for a specific NPU driver to be instantiated
Added: ethosu_deregister_Driver(...) to allow for a specific NPU driver to be de-registered
Added: ethosu_reserve_driver(...) to reserve & return the first NPU driver instance available
Added: ethosu_release_driver(...) to release a specific NPU driver instance and make it available again
Added: *registered_drivers - A  static linked list of drivers ready be used

---ethosu_pmu---
Modified: Declarations for pmu interfaces to support multiNPU (takes *drv)

---ethosu_device---
Modified: Resolved a circular include dependency (Remove include and add ETHOSU_PMU_NCOUNTERS macro)

Change-Id: Iede41cd41bb0d5d483bd9d929d1b6c9ca5d3c48e