Documentation: riscv: Document the sv57 VM layout

Documentation: riscv: Document the sv57 VM layout

RISC-V has been supporting the "sv57" address translation mode for a
while, but is has not been added to the VM layout documentation. Let
us fix that.

Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20221118171556.1612190-1-bjorn@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>