tee: add forward declaration for struct device

tee_drv.h references struct device, but does not include device.h nor
platform_device.h. Therefore, if tee_drv.h is included by some file
that does not pull device.h nor platform_device.h beforehand, we have a
compile warning. Fix this by adding a forward declaration.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>