MAINTAINERS: Add Rob Herring and remove Andy Murray as PCI reviewers

MAINTAINERS: Add Rob Herring and remove Andy Murray as PCI reviewers

Andy Murray decided to step down as PCI controller reviewer and Rob Herring
is willing to help review PCI controller patches.

Update the respective MAINTAINERS entries to reflect this change.

Link: https://lore.kernel.org/r/20200422150336.10528-1-lorenzo.pieralisi@arm.com
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Andrew Murray <amurray@thegoodpenguin.co.uk>