of: add 'const' for of_property_*_string*() parameter '*np'

The of_property_{read,count,match}_string* family of functions never
modify the struct device_node pointer that is passed in, so there is no
reason for it to be non-const. Equivalent functions for all other types
already take a 'const struct device_node *np'.

Signed-off-by: David Rivshin <drivshin@allworx.com>
Signed-off-by: Rob Herring <robh@kernel.org>