powerpc/boot: Convert more files to use SPDX tags

These files are all plain GPL 2.0, with a second sentence about being
licensed as-is.

Similar to the rule in commit 577b61ce ("treewide: Replace GPLv2
boilerplate/reference with SPDX - gpl-2.0_398.RULE").

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220819110430.433984-1-mpe@ellerman.id.au