spi, atmel: move CONFIG_SYS_SPI_WRITE_TOUT into common header

move CONFIG_SYS_SPI_WRITE_TOUT into drivers/spi/atmel_spi.h
and define a default value. Delete this define in the board
config files, where it is possible (all boards use currently
the same value).

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>