SPI HAL: add a lsb_first field in spi_settings

[jnaulet]

Add something like:

bool lsb_first

or maybe better:

typedef enum { SPI_ENDIANNESS_LSB_FIRST, SPI_ENDIANNESS_MSB_FIRST, SPI_ENDIANNESS_COUNT } spi_endianness_t;

in struct spi_settings