staging: mt7621-pci: use gpios for properly reset

staging: mt7621-pci: use gpios for properly reset

Original driver code was using three gpio's for reset
asserts and deasserts the pcis. Instead of using that
a general reset control with a perst gpio was introduced
and it seems it is partially working but sometimes there
are some unexpected hangs on boot. This commit make use of
the three original gpios using 'reset-gpios' property of
the device tree and removes the reset line and perst gpio.
According to the mediatek aplication note v0.1 there are
three gpios used for pcie ports reset control: gpio#19,
gpio#8 and gpio#7 for slots 0, 1 and 2 respectively.
This schema can be used separately for mt7621A but in some
boards due to pin share issue, if the PCM and I2S function
are enable at the same time, there are no enough GPIO to
control per-port PCIe reset. In those cases gpio#19 is enought
for reset the three ports together. Because of this we just
try to get the three gpios but if some of them fail we are not
failing in boot process, just prints a kernel notice and take
after into account if the descriptor is or not valid in order
to use it. All of them are set as GPIO output low configuration.
The gpio descriptor's API takes device tree property into account
and invert value if the pin is configured as active low.
So we also have to properly request pins from device tree
and set values correct in assert and deassert functions.
After this changes the order to make all assert and
deassert in the 'probe' process makes more sense:
* Parse device tree.
* make assert of the RC's and EP's before doing anything else.
* make deassert of the RC's before initializing the phy.
* Init the phy.
* make deassert of the EP's before initialize pci ports.
* Normal PCI initialization.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20200313200913.24321-2-sergio.paracuellos@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>