Replace looping construct with actual block writes.
Transaction in spi_master_block_write funtion is protected by critical
section to ensure it is completed without interruption. A hardware
limitation may cause data corruption if the TX fifo is allowed to run
empty.
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.