reg->test_mux_addr_shift is used only if pdata->test_mux is
non-zero. pdata->test_mux is defined only on Exynos3250 and
Exynos4412 (other SoC types don't even have pdata->test_mux
entry assigned in their struct exynos_tmu_registers instances)
so the abstraction is not needed and can be removed.
There should be no functional changes caused by this patch.