lib: sbi: Add sbi_nputs() function

We add new sbi_nputs() which help us print a fixed number of characters
from a physical memory location.

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