PCI: thunder-pem: Factor out resource lookup

[ Upstream commit 0d414268 ]

Pull the register resource lookup out of thunder_pem_init() so we can
easily add a corresponding lookup using ACPI.  No functional change
intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>