openrisc: Add full instruction cache invalidate functions

openrisc: Add full instruction cache invalidate functions

Add functions to invalidate all cache lines which we will use for
static_key patching.

On OpenRISC there is no instruction to invalidate an entire cache so we
loop and invalidate cache lines one by one.  This is not extremely
expensive on OpenRISC as we usually have only a few hundred cache lines.

I considered using the invalidate cache page or range functions.
However, tracking which ranges need invalidation would have been more
expensive than flushing all pages.

Cc: stable@vger.kernel.org
Signed-off-by: Stafford Horne <shorne@gmail.com>