RISC-V: Enable SBI based earlycon support

Let us enable SBI based earlycon support in defconfig for both RV32
and RV64 so that "earlycon=sbi" can be used again.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20231124070905.1043092-6-apatel@ventanamicro.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>