arm: move SYS_ARCH_TIMER to KConfig

SYS_ARCH_TIMER guards the usage of the ARM Generic Timer (aka arch
timer) in U-Boot.
At the moment it is mandatory for ARMv8 and used by a few ARMv7 boards.
Add a proper Kconfig symbol to express this dependency properly,
allowing certain board configuration to later disable arch timer in case
there are any problems with it.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
[tuomas: rebase + fix conflicts and resync with moveconfig & use select]
Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/arch/arm/mach-stm32mp/Kconfig b/arch/arm/mach-stm32mp/Kconfig
index 8ca97bf..4d59480 100644
--- a/arch/arm/mach-stm32mp/Kconfig
+++ b/arch/arm/mach-stm32mp/Kconfig
@@ -27,6 +27,7 @@
 	select CPU_V7
 	select PINCTRL_STM32
 	select STM32_RESET
+	select SYS_ARCH_TIMER
 	select SYSRESET_SYSCON
 	help
 		target STMicroelectronics SOC STM32MP1 family