mips: pci-mt7620: rework initialization procedure

mips: pci-mt7620: rework initialization procedure

Move the reset operation to the common part to reduce the code
redundancy. They are actually the same and needed for all SoCs.
Disabling power and clock are unnecessary for MT7620 and will be
removed. In vendor SDK, it's used to save the power when the PCI
driver is not selected. The MT7628 GPIO pinctrl has been removed
because this should be done in device-tree. Some delay intervals
have also been increased to follow the recommendations of the SoC
SDK and datasheet. Tested on both MT7620 and MT7628.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>