ARC: [ASID] get_new_mmu_context() refactoring

ARC: [ASID] get_new_mmu_context() refactoring

ASID allocation changes/1

This patch does 2 things:

(1) get_new_mmu_context() moves mm->ASID to a new value ONLY if it was
    from a prev allocation cycle/generation OR if mm had no ASID
    allocated (as opposed to before where it would unconditionally
    assign it a new ASID)

    Callers desiring unconditional update of ASID, e.g.local_flush_tlb_mm()
    (for parent's addess space invalidation at fork) need to first force
    the parent to an unallocated ASID.

(2) get_new_mmu_context() always sets the MMU PID reg with unchanged/new
    ASID value.

The gains are:
- consolidation of all asid alloc logic into get_new_mmu_context()
- avoiding code duplication in switch_mm() for PID reg setting
- Enables future change to fold activate_mm() into switch_mm()

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>