After we switched the two drivers that have .need_valid_mask
set to use the callback for setting up the .valid_mask,
we can just use the presence of the .init_valid_mask()
callback (or the OF reserved ranges, nota bene) to determine
whether to allocate the mask or not and we can drop the
.need_valid_mask field altogether.

Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: Amelie Delaunay <amelie.delaunay@st.com>
Cc: Stephen Boyd <swboyd@chromium.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20190819093058.10863-1-linus.walleij@linaro.org