[CORE] vmm_host_irq: Simplify required architecture dependencies

This patch reduces dependency of vmm_host_irq on architecture code.

It does the following changes:
1. Removes ARCH_HOST_IRQ_COUNT instead introduces arch specific
   menuconfig option named CONFIG_HOST_IRQ_COUNT
2. Removed arch_host_irq_active() instead introduced
   vmm_host_irq_set_active_callback() to register a callback
   which can be used by PIC code to provide a function for
   determining current active irq.
3. Removed per board arch_host_irq.h from everywhere

Due to the above change we have reduced lot of duplicate code
from ARM board suppports. We now require only arch_host_irq_init()
from architecture code.

Also, in future this will help us have a generic DTB based board for
wide range or ARM boards.

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