ARM: mvebu: enable ARM_GLOBAL_TIMER compilation Armada 38x platforms

Armada 38x SoCs along with legacy timer (time-armada-370-xp.c),
comprise generic Cortex-A9 global timer (arm_global_timer.c).
Enable its compilation. The system clocksource subsystem
will pick one of above two available ones in case the global
timer node is present in the device tree.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>