of: add include asm/setup.h in drivers/of/fdt.c

In the file drivers/of/fdt.c, it uses the COMMAND_LINE_SIZE which is stated
in asm/setup.h, so asm/setup.h should be included in drivers/of/fdt.c.

Signed-off-by: Yu Yue <yuyue@mprc.pku.edu.cn>
Signed-off-by: Guan Xuetao <guanxuetao@mprc.pku.edu.cn>

Cc: Grant Likerly <grant.likely@secretlab.ca>
Cc: devicetree-discuss@lists.ozlabs.org
Cc: Arnd Bergmann <arnd@arndb.de>