arm64: Dump kernel memory region surrounding registers.

In the event of a crash, dump the memory surrounding the addresses
pointed to by CPU registers. This may help with the debug of some
memory corruption issues. This is similar to what's done for 32-bit
ARM architecture.

Change-Id: I93a5724568e17a8394c1822ec7245673abae3c24
Signed-off-by: Hanumant Singh <hanumant@codeaurora.org>