RISC-V: Enable second UART on sifive_e and sifive_u

RISC-V: Enable second UART on sifive_e and sifive_u

Previously the second UARTs on the sifive_e and sifive_u machines
where disabled due to check-qtest-riscv32 and check-qtest-riscv64
failures. Recent changes in the QEMU core serial code have
resolved these failures so the second UARTs can be instantiated.

Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Signed-off-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>