RISC-V: defconfig: Enable RISC-V SBI earlycon support

This patch enables RISC-V SBI earlycon support in default defconfig
so that we can use "earlycon=sbi" in kernel parameters for early
debug prints.

Signed-off-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>