riscv: Allow NOMMU kernels to run in S-mode

riscv: Allow NOMMU kernels to run in S-mode

For ease of testing, it is convenient to run NOMMU kernels in supervisor
mode. The only required change is to offset the kernel load address,
since the beginning of RAM is usually reserved for M-mode firmware.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240227003630.3634533-5-samuel.holland@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>