lib: sbi_illegal_insn: Fix FENCE.TSO emulation infinite trap loop

In case of missing "FENCE.TSO" instruction implementation,
opensbi can emulate the "FENCE.TSO" with "FENCE RW,RW", but
mepc was not incremented to continue from the next instruction
causing infinite trap.

Fixes: cb8271c8

 ("lib: sbi_illegal_insn: Add emulation for fence.tso")
Signed-off-by: Rahul Pathak <rpathak@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Samuel Holland <samuel@sholland.org>