virtual int write(const char *tx_buffer, int tx_length,
            char *rx_buffer, int rx_length);

The main benefit of block-level SPI writes is the performance
improvement from not acquiring a mutex lock between each byte sent on
the SPI bus. The block write may also be poked through the hal level for
additional speed improvements.

Poking the block-level SPI writes through the HAL level gives more
freedom to targets to improve SPI transaction speed.

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.

performance improvements:
naive block writes    3.997Mbps
DSPI block writes    17.809Mbps

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.