PCI: mediatek: Add new method to get shared pcie-cfg base address

For the new dts format, add a new method to get
shared pcie-cfg base address and use it to configure
the PCIECFG controller

Link: https://lore.kernel.org/r/20210823032800.1660-3-chuanjia.liu@mediatek.com


Signed-off-by: Chuanjia Liu <chuanjia.liu@mediatek.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>