ARM: sunxi_defconfig: increase the number of maximum number of CPUs to 8

ARM: sunxi_defconfig: increase the number of maximum number of CPUs to 8

The a80 optimus has 8 CPUs. I propose we increase the maximum number of CPUs to 8 to avoid the following warning identified during automated boot testing [1].

------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at ../arch/arm/kernel/devtree.c:144 arm_dt_init_cpu_maps+0x110/0x1e0()
DT /cpu 5 nodes greater than max cores 4, capping them
CPU: 0 PID: 0 Comm: swapper Not tainted 3.19.0-00528-gbdccc4edeb03 #1
Hardware name: Allwinner sun9i Family
[] (unwind_backtrace) from [] (show_stack+0x10/0x14)
[] (show_stack) from [] (dump_stack+0x74/0x90)
[] (dump_stack) from [] (warn_slowpath_common+0x70/0xac)
[] (warn_slowpath_common) from [] (warn_slowpath_fmt+0x30/0x40)
[] (warn_slowpath_fmt) from [] (arm_dt_init_cpu_maps+0x110/0x1e0)
[] (arm_dt_init_cpu_maps) from [] (setup_arch+0x634/0x8d4)
[] (setup_arch) from [] (start_kernel+0x88/0x3ac)
[] (start_kernel) from [<20008074>] (0x20008074)
---[ end trace cb88537fdc8fa200 ]---

[1] http://storage.kernelci.org/mainline/v3.19-528-gbdccc4edeb03/arm-sunxi_defconfig/lab-tbaker/boot-sun9i-a80-optimus.html

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Olof Johansson <olof@lixom.net>
Cc: Kevin Hilman <khilman@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>