ARM: Kconfig: Select clocksource in STM32 entry

STM32 clocksource driver needs to be selected if ARCH_STM32.
Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm/Kconfig

@@ -978,6 +978,7 @@ config ARCH_STM32
 	depends on ARM_SINGLE_ARMV7M
 	select ARCH_HAS_RESET_CONTROLLER
 	select ARMV7M_SYSTICK
+	select CLKSRC_STM32
 	select RESET_CONTROLLER
 	help
 	  Support for STMicroelectronics STM32 processors.