Revert "arm64: a37xx: pci: Assert PERST# signal when unloading driver"

This reverts commit 828d3262.

This change revers code which asserting PERST# signal when unloading
driver. Driver's remove callback is still there as it is used for other
functionality.

Asserting PERST# signal prior booting kernel is causing that A3720 boards
(Turris MOX and Espressobin) with stable Linux kernel versions 4.14 and
4.19 are not able to detect some PCIe cards (e.g. Compex WLE200 and WLE900)
and anymore. When PERST# signal is not asserted these cards are detected
correctly. As this is regression for existing stable Linux kernel versions
revert this problematic change in U-Boot.

To make cards working with OpenWRT 4.14 kernel it is needed to disable link
training prior booting kernel, which is already done in driver's remove
callback.

Described issue is in Linux kernel pci aardvark driver which is (hopefully)
fixed in latest upstream versions. Latest upstream versions should be able
to initialize PCIe bus and detects cards independently of the link training
and PERST# signal state.

So with this change, U-Boot on A3720 boards should be able to boot OpenWRT
4.14 kernel, stable 4.14 and 4.19 kernels and also latest mainline kernels.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>