soc: mediatek: mtk-svs: Add explicit include for cpu.h

soc: mediatek: mtk-svs: Add explicit include for cpu.h

Removing the include of cpu.h from of_device.h (included by
of_platform.h) causes an error:

drivers/soc/mediatek/mtk-svs.c:2134:41: error: implicit declaration of function 'get_cpu_device'; did you mean 'get_swap_device'? [-Werror=implicit-function-declaration]

of_platform.h is still needed for of_find_device_by_node().

Link: https://lore.kernel.org/r/20230329-dt-cpu-header-cleanups-v1-13-581e2605fe47@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>