[ARM] Kirkwood: enable PCIe before reading device ID register

PCIe may have been disabled (by kirkwood_clock_gate)
if this kernel was started by kexec.  Make sure PCIe
is enabled before attempting to access the device ID
register, otherwise the system will hang.

Signed-off-by: Eric Cooper <ecc@cmu.edu>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>