riscv: Fix Kconfig indentation

Adjust indentation from spaces to tab (+optional two spaces) as in
coding style with command like:
	$ sed -e 's/^        /\t/' -i */Kconfig

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
[paul.walmsley@sifive.com: use two leading spaces for help text to align
 with common arch/ practice]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>