MAINTAINERS: Add DesignWare, i.MX6, Armada, R-Car PCI host maintainers

Add entries for PCI host controller drivers in drivers/pci/host/.

Signed-off-by: Mohit Kumar <mohit.kumar@st.com>             # DESIGNWARE
Signed-off-by: Pratyush Anand <pratyush.anand@st.com>       # DESIGNWARE
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>         # R-CAR
Double-Plus-Acked-by: Jason Cooper <jason@lakedaemon.net>   # MVEBU
Acked-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Richard Zhu <r65037@freescale.com>                # IMX6