platsupport: SPI API: Add an "ID" member to SPI interface

pragma once and for all

Fix whitespace

- remove trailing whitespace
- remove duplicate blank lines
- remove blank lines at end of file

fix licenses

libplatsupport: Unify Chip select enum variants

Was introduced in pr #79. Previously these were defined per platform,
and when defining globally for TK1 there was a name conflict with
exynos.

libplatsupport: add chipselect function injection

The TK1 spi driver is currently only used in a CAmkES application that
was managing the chip select functionality at the component level. This
commit moves the CS functionality into the driver by allowing the CAmkES
application to initialise the driver with the cs function pointer that
the driver can then use more intelligently.

Remove trailing whitespace

libplatsupport: Mark an argument to spi_prepare_transfer as `const`.

This avoids spurious warnings when passing in a pointer to a `const` struct.