arm64: smp: Add missing prototype for some smp.c functions

arm64: smp: Add missing prototype for some smp.c functions

In commit c598c618dc9c
("arm64: Support arch_irq_work_raise() via self IPIs") a new
function "arch_irq_work_raise" was added without a prototype.

In commit ad2aae9597fa
("arm64: Implement panic_smp_self_stop()") a new
function "panic_smp_self_stop" was added without a prototype.

We get the following warnings on W=1:
arch/arm64/kernel/smp.c:842:6: warning: no previous prototype
for ‘arch_irq_work_raise’ [-Wmissing-prototypes]
arch/arm64/kernel/smp.c:862:6: warning: no previous prototype
for ‘panic_smp_self_stop’ [-Wmissing-prototypes]

Fix the warnings by:
1. Adding the prototype for 'arch_irq_work_raise' in irq_work.h
2. Adding the prototype for 'panic_smp_self_stop' in smp.h

Signed-off-by: Chen Lifu <chenlifu@huawei.com>
Link: https://lore.kernel.org/r/20210329034343.183974-1-chenlifu@huawei.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>