clockevents: asm9260: Fix compilation error with sparc/sparc64 allyesconfig

The Kconfig options for the asm9260 timer is wrong as it can be selected by
another platform with allyes config and thus leading to a compilation failure
as some non arch related code is pulled by the compilation.

Fix this by having the platform Kconfig to select the timer as it is done for
the others drivers.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Oleksij Rempel <linux@rempel-privat.de>

Conflicts:
	drivers/clocksource/Kconfig