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

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Wenyou Yang <wenyouya@gmail.com>