ARM: better diagnostics with missing/corrupt dtb

With a kernel containing both DT and atag support, the diagnostics
output when the dtb is missing or corrupt assume that we're trying
to boot using atags and the machine ID, and only print the machine
ID.  This is not useful for diagnosing a missing or corrupt dtb.

Move the message into arch/arm/kernel/setup.c, and print the address
of the dtb/atag list, and the first 16 bytes of memory of the dtb or
atag list.

This allows us to see whether the dtb was corrupted in some way,
causing the fallback to the machine ID / atag list.

Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>