openrisc: avoid using function parameter regs in reset vector

The kernel might be invoked through the reset vector, so to
preserve parameters passed to it, temp regs that are not
in the function parameter range needs to be used.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Signed-off-by: Jonas Bonn <jonas@southpole.se>