regmap: zero out the regmap on allocation

Some fields will be introduced in the regmap structure that should be
set to 0 by default. So, once we allocate a regmap, make sure it is
zeroed out to avoid unexpected defaults for those values.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>