spi: introduce `struct spi_dt_spec`

Introduces the `struct spi_dt_spec` type, which contains the complete
SPI bus information derived from devicetree. It serves the same purpose
as `struct gpio_dt_spec` in that it can be constructed automatically in
`DEVICE_DT_INST_DEFINE` macros and provided as a single handle to SPI
API calls. Includes a single function, `spi_is_ready` that validates all
SPI bus dependencies are ready.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>