lib: sbi: Introduce the SBI debug triggers extension support

RISC-V Debug specification includes Sdtrig ISA extension
which describes Trigger Module. Triggers can cause
a breakpoint exception or trace action without execution
of a special instruction. They can be used to implement
hardware breakpoints and watchpoints for native debugging.

The SBI Debut Trigger extension (Draft v6) can be found at:
https://lists.riscv.org/g/tech-debug/topic/99825362#1302



This patch is an initial implementation of SBI Debug
Trigger Extension (Draft v6) in OpenSBI.

The following features are supported:
 * mcontrol, mcontrol6 triggers
 * Breakpoint and trace actions

NOTE: Chained triggers are not supported

Signed-off-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>