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>