PCI: mediatek: Use readl_poll_timeout() to wait for Gen2 training

Wait for Gen2 training with readl_poll_timeout(), and simplify the hardware
assert logical by merging it into a new mtk_pcie_startup_port() interface.

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>