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