of/pci: fix sleeping from atomic context in pci_register_io_range()

pci_register_io_range() holds a spinlock while calling kzalloc() with
GFP_KERNEL. Avoid this by using GFP_ATOMIC.

Change-Id: I49a6a2b89ed797770413e0a52cea3506a1663d79
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>