rtc: rtc7301: Support byte-addressed IO

rtc: rtc7301: Support byte-addressed IO

The old RTC7301 driver in OpenWrt used byte access, but the
current mainline Linux driver uses 32bit word access.

Make this configurable using device properties using the
standard property "reg-io-width" in e.g. device tree.

This is needed for the USRobotics USR8200 which has the
chip connected using byte accesses.

Debugging and testing by Howard Harte.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Akinobu Mita <akinobu.mita@gmail.com>
Link: https://lore.kernel.org/r/20231010-rtc-7301-regwidth-v3-2-ade586b62794@linaro.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>