lib: sbi: Detect extensions from the ISA string in DT

Enable access to some extensions through menvcfg and show them in "Boot
HART ISA Extensions" if they are present in the device tree.

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