PCI: designware-ep: Make dw_pcie_ep_reset_bar() handle 64-bit BARs properly

Since a 64-bit BAR consists of a BAR pair, we need to write to both
BARs in the BAR pair to clear the BAR properly.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>