of: fdt: Constify 'pathp'

Constify 'pathp' in order to get rid of the following warning:

drivers/of/fdt.c:491:10: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
[Rob Herring: also constify np and lp]
Signed-off-by: Rob Herring <rob.herring@calxeda.com>