[ARM] Make sp804 timer self initializing

We use newly introduced macros VMM_CLOCKSOURCE_INIT_DECLARE() and
VMM_CLOCKCHIP_INIT_DECLARE() to make sp804 timer code self initializing.

This means we don't need explictily initialize sp804 timer from board
specific code.

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