lib: sbi: Add sbi_trap_set_external_irqfn() API

This patch adds sbi_trap_set_external_irqfn() API which can be used by
OpenSBI platform code to set a callback function for external interrupts.
The RISC-V AIA IMSIC driver will use this API to implement inter-processor
interrupts on-top-of MSIs.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>