MAINTAINERS: Add Sam as the maintainer for NCSI

Sam has been handing the maintenance of NCSI for a number release cycles
now.

Acked-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>