Consolidate of_find_property

The only change here is that a readlock is taken while the property list
is being traversed on Sparc where it was not taken previously.

Also, Sparc uses strcasecmp to compare property names while PowerPC
uses strcmp.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Paul Mackerras <paulus@samba.org>
Acked-by: David S. Miller <davem@davemloft.net>