pwm: renesas-tpu: Enable driver compilation with COMPILE_TEST

This helps increasing build testing coverage.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: linux-pwm@vger.kernel.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>