Ignore phandle properties in /aliases

The 'alias_paths' check verifies that each property in /aliases is a valid
path to another node.  However this can cans false positives trees where
the /aliases node has a phandle property, which isn't in this format but
is allowed.  In particular this situation can be common with trees dumped
from some real OF systems (which typically generate a phandle for every
node).

Special case this to avoid the spurious error.

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