m68k: defconfig: Enable ICY I2C and LTC2990 on Amiga

Enable support for the ICY I2C board for Amiga, which is typically
equipped with an LTC2990 hwmon chip, in the Amiga and multi-platform
defconfig files.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20191021070438.10819-1-geert@linux-m68k.org
Acked-by: Max Staudt <max@enpas.org>