target/hppa: Replace MMU_PHYS_IDX with MMU_ABS_IDX, MMU_ABS_W_IDX

Align the language with pa2.0, separating absolute and physical.
The translation from absolute to physical depends on PSW.W, and
we prefer not to flush between changes, therefore use 2 mmu_idx.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>