dt: add 'const' for of_property_read_string parameter **out_string

The existing dt codes usually call of_get_property to get a string
property and save it as a 'const char *'.  The patch adds'const' for
of_property_read_string parameter **out_string to make the converting
of existing code a little easier.

Signed-off-by: Shawn Guo <[email protected]>
Signed-off-by: Grant Likely <[email protected]>
2 files changed