lib: Add hfence instruction encoding

Currently, the toolchains do not have support for hfence instruction.
Hence, the instruction are hardcode until we have toolchain support.

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

lib/sbi/objects.mk

@@ -16,6 +16,7 @@ libsbi-objs-y += sbi_console.o
 libsbi-objs-y += sbi_ecall.o
 libsbi-objs-y += sbi_emulate_csr.o
 libsbi-objs-y += sbi_fifo.o
+libsbi-objs-y += sbi_hfence.o
 libsbi-objs-y += sbi_hart.o
 libsbi-objs-y += sbi_illegal_insn.o
 libsbi-objs-y += sbi_init.o