powerpc: Update asm-prototypes.h comment

This header was recently cleaned up in commit 76222808 ("powerpc:
Move C prototypes out of asm-prototypes.h"), update the comment to
reflect it's proper purpose.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220617080243.2177583-1-mpe@ellerman.id.au