clocksource: samsung_pwm_timer: Let platforms select the driver

This patch modifies the way of enabling the driver to let the platforms
select it in their Kconfig instead of specifying particular platforms in
Kconfig entry of the driver.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Olof Johansson <olof@lixom.net>