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-qemu/Kconfig b/arch/arm/mach-qemu/Kconfig
index 133163a..226dfa3 100644
--- a/arch/arm/mach-qemu/Kconfig
+++ b/arch/arm/mach-qemu/Kconfig
@@ -16,6 +16,7 @@
 	depends on ARCH_QEMU
 	select CPU_V7
 	select ARCH_SUPPORT_PSCI
+	select SYS_ARCH_TIMER
 
 config TARGET_QEMU_ARM_64BIT
 	bool "Support qemu_arm64"