kbuild: add "Section mismatch" warning whitelist for powerpc

This patch fixes the following class of "Section mismatch" warnings when
building powerpc platforms.

WARNING: arch/powerpc/kernel/built-in.o - Section mismatch: reference to .init.data:.got2 from prom_entry (offset 0x0)
WARNING: arch/powerpc/platforms/built-in.o - Section mismatch: reference to .init.text:mpc8313_rdb_probe from .machine.desc after 'mach_mpc8313_rdb' (at offset 0x4)
....

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>