powerpc/64: Correct comment on LOAD_HANDLER()

The comment for LOAD_HANDLER() was wrong. The part about kdump has not
been true since 1f6a93e4 ("powerpc: Make it possible to move the
interrupt handlers away from the kernel").

Describe how it currently works, and combine the two separate comments
into one.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nick Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>