lib: sbi: Add sbi_ngets() function

We add new sbi_ngets() which help us read characters into a
physical memory location.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>