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>

drivers/clocksource/Kconfig

@@ -69,7 +69,7 @@ config CLKSRC_METAG_GENERIC
 	  This option enables support for the Meta per-thread timers.
 
 config CLKSRC_SAMSUNG_PWM
-	def_bool ARCH_EXYNOS4
+	bool
 	depends on OF
 	select CLKSRC_MMIO
 	help