lib: irqchip/imsic: configure mstateen

When mstateen registers are implemented, the AIA related
configurations need to be done in mstateen for the IMSIC
initialization to succeed.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>