Obsolete guidance doc and reset to default RISC-V template

The ACPI guidance document which was hosted here was never frozen/ratified.
Instead, the relavant contents are created in BRS TG as non-normative
document [1]. So, reset this repo to default RVI template and remove
existing content so that it doesn't cause confusion with BRS TG's spec.

[1] - https://github.com/riscv-non-isa/riscv-brs/blob/main/non-normative/acpi.adoc



Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>