[CORE] Increase size of Sync IPI fifo and Async IPI fifo

The Sync & Async IPI fifo were little small for SMP host with
four CPUs (i.e. VExpress-A9 or VExpress-A15).

For now, we just increase this fifo sizes to some reasonable
value with some explanation. If things keep varying from one
SMP host to another then we will also have a config option
for IPI fifo sizes.

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