MAINTAINERS: add myself as Samsung SPI maintainer

Create a new entry for the Samsung SPI driver supported by the
drivers/spi/spi-s3c* and remove it from its original place under
"ARM/SAMSUNG EXYNOS ARM ARCHITECTURES".

The original maintainership inherited from the Samsung Exynos
ARM Architecture is kept as it was (i.e. Kukjin and Krzysztof), I
will help and co-maintain the driver.

Signed-off-by: Andi Shyti <andi.shyti@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>