[PATCH] powerpc: fix of_parse_dma_window

My js20 appears to lack the ibm,#dma- properties, and boot fails with a
"Kernel panic - not syncing: iommu_init_table: Can't allocate 0 bytes"
message.

This adds a fallback to the "#address-cells" property in case the
"#ibm,dma-address-cells" property is missing.   Tested on js20 and
power5 lpar.

Unless there is a more elegant solution... :-)

Signed-off-by: Will Schmidt <willschm@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>