Implement K60 SPI driver

This PR implements a common SPI bus master driver for the K60.
All Mulle SPI device drivers are converted to the common API, except the radio, which will be added in another PR.
cmd_rdsr() is static and only used in one place more than this (at line 450).

consider moving the call to spi_acquire_bus() outside of cmd_rdsr() to avoid this looped release-acquire mess ?