lib: Optimize unpriv load/store implementation

This patch optimize unpriv load/store implementation by having
dedicated unpriv trap handler (just like KVM RISC-V).

As a result of this optimization:
1. We have reduced roughly 13+ instruction in all unpriv load/store
   functions. The reduced instruction also include two function calls.
2. Per-HART trap info pointer in scratch space is now redundant
   hence removed.
3. The sbi_trap_handler() is now much cleaner because we don't have
   to handle unpriv load/store traps.

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