clocksource/drivers/renesas-ostm: Convert to SPDX identifiers

This patch updates license to use SPDX-License-Identifier instead of verbose
license text.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>