clocksource: samsung_pwm_timer: Do not use clocksource_mmio

clocksource: samsung_pwm_timer: Do not use clocksource_mmio

In case of Samsung PWM timer, clocksource MMIO can not be used, because
custom suspend/resume callbacks are required.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Mark Brown <broonie@linaro.org>
Tested-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>