riscv: Kconfig: Fix indentation and add comments

The convention for indentation seems to be a single tab. Help text is
further indented by an additional two whitespaces. Fix the lines that
violate these rules.

While add it, add trailing comments to endmenu statements for better
readability.

Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Link: https://lore.kernel.org/r/20220520120232.148310-2-juergh@canonical.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>