lib: sbi_trap: Simplify sbi_trap_handler() API

This patch simplify sbi_trap_handler() API as follows:
1. Remove current hartid local variable because sbi_trap_handler()
   itself does not need it.
2. Remove scratch parameter because none of the functions directly
   called by sbi_trap_handler() require it.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>