[ARM] Make generic timer self initializing

We use newly introduced VMM_CLOCKSOURCE_INIT_DECLARE() and
VMM_CLOCKCHIP_INIT_DECLARE() macros for declaring init
functions of generic timer.

Now we don't need explictily initialize generic timer from
board specific code. In fact, we can totally remove the
arch_clockchip_init() and arch_clocksource_init() functions
for generic-v8 board.

Signed-off-by: Anup Patel <anup@brainfault.org>