firmware: arm_sdei: Prohibit probing in '_sdei_handler'

Functions called in '_sdei_handler' are needed to be marked as
'nokprobe'. Because these functions are called in NMI context and
neither the arch-code's debug infrastructure nor kprobes core supports
this.

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>