RISC-V: Add auxiliary files

RISC-V: Add auxiliary files

To support a new architecture we need to provide a few helper functions
for memory, cache, timer, etc support.

This patch adds the remainders of those. Some bits are still disabled,
as I couldn't guarantee that we're always running on models / in modes
where the respective hardware is available.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>