arm64: a37xx: pci: Disable link training when unloading driver

As required by PCI Express spec a delay for at least 100ms after
de-asserting PERST# signal is needed before link training is enabled.

Linux kernels prior to 5.8 version do not automatically disable link
training before de-asserting PERST# signal, therefore this requirement is
not fulfilled.

Above requirement is needed for proper detection of some Compex PCIe WiFi
cards. Otherwise Linux kernel cannot detect it.

To allow using those PCIe cards with older Linux kernel versions booted by
U-Boot compiled with U-Boot a37xx pci driver, disable link training in
U-Boot when unloading this pci driver.

Thanks to DM_FLAG_OS_PREPARE flag, U-Boot automatically unload this driver
when booting Linux kernel.

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