lib/fdt: Constify input-only char* arguments

[lnksz]

The functions are called with constant character literals aka const char *

ret = of_get_node_offset(blob, "chosen", &nodeoffset);
ret = of_get_node_offset(blob, "memory", &nodeoffset);
ret = of_set_property(blob, nodeoffset, "bootargs", value, valuelen);
ret = of_set_property(blob, nodeoffset, "device_type", "memory", sizeof("memory"));
ret = of_set_property(blob, nodeoffset, "reg", data, valuelen);

[ehristev]

Hi, Could you please edit your commit message and add your Signed-off-by tag ? Thanks

[lnksz]

Sorry, I tried out the online edit-commit function of github. I re-created this in #112 from the command line...