regmap: Introduce init_range

Both fdtdec_get_addr_size_fixed and of_address_to_resource can fail with
an error, which is not currently checked during regmap initialization.

Since the indentation depth is already quite deep, extract a new
'init_range' method to do the initialization.

Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Mario Six <mario.six@gdsys.cc>