MIPS: Fix CONFIG_OF_EARLY_FLATTREE=n builds

Configurations with CONFIG_OF_EARLY_FLATTREE=n fail to build since
commit 7784cac6 ("MIPS: cmdline: Clean up boot_command_line
initialization") because of_scan_flat_dt() & of_scan_flat_dt() are not
defined in these configurations. Fix this by #ifdef'ing the affected
code...

Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: 7784cac6 ("MIPS: cmdline: Clean up boot_command_line initialization")
Reported-by: kbuild test robot <lkp@intel.com>
Cc: linux-mips@vger.kernel.org