timer: Add timer_start_us() API
nrf51822/timer: Implement timer_start_us() API

This patch implements the timer_start_us() API. Due to a limitation in
Nordic's SDK, the minimum time allowed is 150 us. If the user pass a
value lower than 150, -EINVAL will be returned.
ll: Add ll_set_advertising_data() API
ll: Add ll_set_scan_response_data() API
bci: Add bci_set_scan_response_data() API
timer: Add TIME_EXP() macro