kexec/kdump: minor Documentation updates for arm64 and Image

Minor updates in Documentation for arm64 as relocatable kernel.  Also
this patch updates documentation for using uncompressed image "Image"
which is used for ARM64.

Link: http://lkml.kernel.org/r/1495104793-6563-1-git-send-email-Bharat.Bhushan@nxp.com


Signed-off-by: Bharat Bhushan <Bharat.Bhushan@nxp.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Pratyush Anand <panand@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>