spi: reorganize drivers

spi: reorganize drivers

Sort the SPI makefile and enforce the naming convention spi_*.c for
spi drivers.

This change also rolls the contents of atmel_spi.h into the .c file
since there is only one user of that particular include file.

v2: - Use 'spi-' prefix instead of 'spi_' to match what seems to be
      be the predominant pattern for subsystem prefixes.
    - Clean up filenames in Kconfig and header comment blocks

Signed-off-by: Grant Likely <[email protected]>
Acked-by: Wolfram Sang <[email protected]>
Acked-by: Linus Walleij <[email protected]>