[ARM] Make GIC self initializing

We use newly introduced VMM_HOST_IRQ_INIT_DECLARE() macro for
declaring init function of GIC driver.

Now we don't need to explictily initialize GIC from board specific
code. In fact, we can totally remove the arch_host_irq_init()
function for boards using GIC.

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