lib: sbi: Implement SBI debug console extension

We implement SBI debug console extension as one of the replacement
SBI extensions. This extension is only available when OpenSBI platform
provides a console device to generic library.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>