regmap: Improve error handling

regmap: Improve error handling

ofnode_read_simple_addr_cells may fail and return a negative error code.
Check for this when initializing regmaps.

Also check if both_len is zero, since this is perfectly possible, and
would lead to a division-by-zero further down the line.

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