Documentation: riscv: Update boot image header since EFI stub is supported

The EFI stub is supported on RISC-V so update the documentation that
explains how the boot image header was reused to support it.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230817130734.10387-3-alexghiti@rivosinc.com