PCI: mediatek: Switch to use platform_get_resource_byname()

This is a transitional patch.  We currently use platfarm_get_resource() for
retrieving the IOMEM resources, but there might be some chips don't have
subsys/shared registers part, which depends on platform design, and these
will be introduced in further patches.

Switch this function to use the platform_get_resource_byname() so that the
binding can be agnostic of the resource order.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>