MIPS: generic: Support booting with built-in or appended DTB

The plat_get_fdt() checked that the kernel was booted using UHI before
reading the 'fw_passed_dtb' variable. However, this variable is also set
when the DT has been appended, or when it has been built into the kernel.

Support these usecases by removing the UHI check.

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