device_tree: Make FDT property data non-const

FDT property data should not be const -- sometimes we need to update it,
for example when fixing up phandles in an overlay. On the other hand
it's occasionally desirable to put a string constant in there without
having to strdup() it all the time... let's just live with the tiny
implicit assumption that the data we'd want to modify (phandle
references, mostly) will never be added from string constants, and put a
cast in dt_add_string_prop().

Change-Id: Ifac103fcff0520cc427ab9a2aa141c65e12507ac
Signed-off-by: Julius Werner <jwerner@chromium.org>
---
M src/include/device_tree.h
M src/lib/device_tree.c
M src/soc/cavium/cn81xx/soc.c
3 files changed, 5 insertions(+), 5 deletions(-)