mfd: syscon: Add a DT property to set value width

Currently syscon has a fixed configuration of 32 bits for register and
values widths. In some cases, it would be desirable to be able to
customize the value width.

For example, certain boards (like the ones manufactured by Technologic
Systems) have a FPGA that is memory-mapped, but its registers are only
16-bit wide.

This patch adds an optional "reg-io-width" DT binding for syscon that
allows to change the width for the data bus (i.e. val_bits). If this
property is provided, it will also set the register stride to
reg-io-width's value. If not provided, the default configuration is
used.

Signed-off-by: Damien Riegel <damien.riegel@savoirfairelinux.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>