fdt_support: board_fdt_chosen_bootargs() should return const char*

fdt_support: board_fdt_chosen_bootargs() should return const char*

It should be structured this way to demonstrate to the caller that
freeing the return value is unnecessary and that the caller cannot
modify it.
The function fdt_setprop() includes a parameter with a const char*
prototype, so it is better to use the const qualifier.

Signed-off-by: Dmitry Rokosov <ddrokosov@salutedevices.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>