MAINTAINERS: add entry for DCP-based trusted keys

This covers trusted keys backed by NXP's DCP (Data Co-Processor) chip
found in smaller i.MX SoCs.

Signed-off-by: David Gstir <david@sigma-star.at>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>