openrisc: head: refactor out tlb flush into it's own function

This brings it inline with the other setup oprations done like the cache
enables _ic_enable and _dc_enable.  Also, this is going to make it
easier to initialize additional cpu's when smp is introduced.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
[shorne@gmail.com: Added commit body]
Signed-off-by: Stafford Horne <shorne@gmail.com>