openrisc: Use asm-generic's version of fix_to_virt() & virt_to_fix()

Openrisc's implementation of fix_to_virt() & virt_to_fix() share same
functionality with ones of asm generic.

Plus, generic version of fix_to_virt() can trap invalid index at compile
time.

Thus, Replace the arch-specific implementations with asm generic's ones.

Signed-off-by: Dawei Li <set_pte_at@outlook.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>