clocksource/drivers/renesas-ostm: Convert to timer_of

Convert the Renesas OSTM driver to use the timer_of framework.
This reduces the driver object size by 367 bytes (with gcc 7.4.0).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191016144747.29538-4-geert+renesas@glider.be

drivers/clocksource/Kconfig

@@ -528,6 +528,7 @@ config SH_TIMER_MTU2
 config RENESAS_OSTM
 	bool "Renesas OSTM timer driver" if COMPILE_TEST
 	select CLKSRC_MMIO
+	select TIMER_OF
 	help
 	  Enables the support for the Renesas OSTM.