rtc: rewrite isl1208 to support DM

Adds devicemodel support to the ISL1208 driver.
This patch drops the non-dm API as no board was using it anyway.
Also add it to Kconfig.

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