openrisc/fault: Fix symbol scope warnings

Sparse reported the following warning:

    arch/openrisc/mm/fault.c:27:15: warning: symbol 'pte_misses' was not declared. Should it be static?
    arch/openrisc/mm/fault.c:28:15: warning: symbol 'pte_errors' was not declared. Should it be static?
    arch/openrisc/mm/fault.c:33:16: warning: symbol 'current_pgd' was not declared. Should it be static?

This patch fixes these by:
 - Remove unused pte_misses and pte_errors counters which are no longer
   used.
 - Add asm/mmu_context.h include to provide the current_pgd declaration.

Signed-off-by: Stafford Horne <shorne@gmail.com>