MAINTAINERS: Add myself as co-maintainer for s5p-sss.c

Add myself as co-maintainer for Samsung Security SubSystem driver.
I have added major functionality to the driver [hash acceleration],
I have access to documentation and to hardware for testing, I can
also dedicate some of my paid time for reviewing and verifying changes
to the driver.

Signed-off-by: Kamil Konieczny <k.konieczny@partner.samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>