MAINTAINERS: Add missing documentation references to PCI Endpoint Subsystem

Adds documentation reference created by Kishon Abraham to the
MAINTAINERS list relative with the PCI endpoint subsystem section.

Link: https://lore.kernel.org/r/4fa78c7a24e8f8ec3206e1e8960dc18f505c9e29.1597695880.git.gustavo.pimentel@synopsys.com


Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>