generic: fix appended dtb support

Appended DTB support is mostly intended to be used on legacy systems,
but it is a valid feature that can be enabled for generic platform,
which currently doesn't support it - if selected, the appended DTB will
be ignored by the platform startup code.

During kernel startup, the appended DTB's location is stored in
fw_passed_dtb if the init code finds what appears to be a valid DTB.
Otherwise (if a0 == -2), a1 is stored in fw_passed_dtb, so either way it
will always point to either a user-passed DTB or built-in DTB.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@fungible.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org