configs: gurnard: Move CONFIG_ATMEL_SPI to defconfigs

Now CONFIG_ATMEL_SPI is defined in Kconfig, so move the
same into defconfig file.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Wenyou Yang <wenyouya@gmail.com>