powerpc: Make dummy section a valid note header

We are declaring the dummy section (used to work around a binutils
bug) as PT_NOTE, but we don't have enough bytes for it to be a valid
note header, and kexec userspace complains:

Warning: Elf Note name is not null terminated
Warning: append= option is not passed. Using the first kernel root partition
Warning: Elf Note name is not null terminated

Instead of using the arbitray value 0xf177 (aka "fill"), declare a
no-name no-description note of type 0.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>