Remove dependency on us_ticker HAL apis for non USTICKER targets
Remove dependency on us_ticker HAL apis for non USTICKER targets

- Surround mbed_us_ticker_api.c with if DEVICE_USTICKER
- get_us_ticker_data() returns NULL for non-usticker targets.

andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed