spi: atmel: Drop atmel_spi.h

atmel_spi.h has register offsets, and atmel_spi_slave
structure, move it into .c file for better readability
and drop atmel_spi.h

Cc: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>