PCI: mediatek: Convert to use pci_host_probe()

Part of mtk_pcie_register_host() is an open-coded version of
pci_host_probe(). So instead of duplicating this code, use
pci_host_probe() directly and remove mtk_pcie_register_host().

Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[lorenzo.pieralisi@arm.com: commit log changes]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Ryder Lee <ryder.lee@mediatek.com>