ARM/PCI: Replace panic with WARN messages on failures

In the ARM PCI bios32 layer, failures to dynamically allocate pci_sys_data
for a PCI bus, or a PCI bus scan failure have to be considered serious
warnings but they should not trigger a system panic so that at least the
system is given a chance to be debugged.

This patch replaces the panic statements with WARN() messages to improve
error reporting in the ARM PCI bios32 layer.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jingoo Han <jingoohan1@gmail.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
CC: Russell King <linux@arm.linux.org.uk>
CC: Marc Zyngier <marc.zyngier@arm.com>