lib: sbi: Improve the code of privilege mode and extensions detection

We can enhance the code by creating 2 unified interface with macro  for
privilege mode and extensions detection, which relies on supported
privilege modes and CSRs.

Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>