MIPS: ingenic: Use fw_passed_dtb even if CONFIG_BUILTIN_DTB

The fw_passed_dtb is now properly initialized even when
CONFIG_BUILTIN_DTB is used, so there's no need to handle it in any
particular way here.

Note that the behaviour is slightly different, as the previous code used
the built-in Device Tree unconditionally, while now the built-in Device
Tree is only used when the bootloader did not provide one.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>