integrator: fix Linux boot failure by emulating dbg region

Commit 9b8c69243 (since reverted) broke the ability to boot the kernel
as the value returned by unassigned_mem_read returned non-zero and left
the kernel looping forever waiting for it to change (see
integrator_led_set in the kernel code).

Relying on a varying implementation detail is incorrect anyway so this
introduces a basic stub of a memory region for the debug/LED section
on the integrator board.

Signed-off-by: Alex Bennée <alex@bennee.com>
Message-id: 1382451366-9539-1-git-send-email-alex.bennee@linaro.org
[PMM: removed three unused fields from struct IntegratorDebugState]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
