libfdt: Don't use 'index' as a local variable name

Using 'index' as a local variable name shadows the standard library index()
function.  This causes warnings on at least some compiler versions.  The
recently added overlay code has a number of instances of this.

This patch replaces 'index' with 'poffset', since 'index' is being used to
mean "offset within a property value" in these cases.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>