cmd:spl:fix: Prevent from a build error on boards, which don't support FDT

Do not compile in FDT related code, when it is not supported.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Stefano Babic <sbabic@denx.de>