Simplify tickers to stop emulating 1us ticks by using native timer resolutions.
Implement ticker get info functions.
Separate rtc and lp ticker init status by implementing separate functions.
Add mbed_critical include file to disable compiler warning messages.
There is an easy default implementation of spi_master_block_write that
just calls spi_master_write in a loop, so the default implementation
of spi_master_block_write has been added to all targets.