ARM: multi_v7_defconfig: Enable support for Samsung S3C64XX SPI

Build the spi-s3c64xx driver as a module to have support for
the SPI controller in several Samsung SoCs.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>