ARCv2: mm: TLB Miss optim: SMP builds can cache pgd pointer in mmu scratch reg

ARCv2: mm: TLB Miss optim: SMP builds can cache pgd pointer in mmu scratch reg

ARC700 exception (and intr handling) didn't have auto stack switching
thus had to rely on stashing a reg temporarily (to free it up) at a
known place in memory, allowing to code up the low level stack switching.
This however was not re-entrant in SMP which thus had to repurpose the
per-cpu MMU SCRATCH DATA register otherwise used to "cache" the task pdg
pointer (vs. reading it from mm struct)

The newer HS cores do have auto-stack switching and thus even SMP builds
can use the MMU SCRATCH reg as originally intended.

This patch fixes the restriction to ARC700 SMP builds only

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