PCI: mvebu: Convert to use devm_ioremap_resource

Commit 75096579 ("lib: devres: Introduce devm_ioremap_resource()")
introduced devm_ioremap_resource() and deprecated the use of
devm_request_and_ioremap().

While at it, modify mvebu_pcie_map_registers() to propagate error code.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>