riscv: Add sfence.vma after early page table changes

This invalidates local TLB after modifying the page tables during early init as
it's too early to handle suprious faults as we otherwise do.

Fixes: f2c17aab

 ("RISC-V: Implement compile-time fixed mappings")
Reported-by: Syven Wang <syven.wang@sifive.com>
Signed-off-by: Syven Wang <syven.wang@sifive.com>
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
[Palmer: Cleaned up the commit text]
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>