rtc: Add read8 and write8 support to isl1208 driver

rtc: Add read8 and write8 support to isl1208 driver

This can be used for device register access from board code.

This allows access to capabilities in the RTC chip not abstracted in
U-Boot's RTC class.  E.g., device NVRAM or a tamper detection circuit.

Cc: Klaus Goger <klaus.goger@theobroma-systems.com>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>