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

This change ensures that PCIe card is put into reset state when U-Boot
stops using it.

DM_FLAG_OS_PREPARE ensures that U-Boot executes driver's remove callback
prior booting Linux kernel.

Linux kernel pci-aardvark driver needs to reset PCIe card via PERST# signal
prior initializing it. If it does not issue reset then some PCIe cards
(specially Compex WiFi cards) are not detected at all.

Putting PCIe card into reset state prior booting Linux kernel would ensure
that card would be properly reset at time when Linux kernel starts
initializing pci-aardvark driver.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Konstantin Porotchkin <kostap@marvell.com>
Tested-by: Andre Heider <a.heider@gmail.com>