i2c: silabs: Fix I2C target crash on long ISR execution

Long I2C target callback functions could lead to an unresponsive
I2C peripheral. This commit ports the official ISR implementation
from Silicon labs to Zephyr:
https://github.com/SiliconLabs/peripheral_examples/blob/master/series2/i2c/i2c_follower/src/main.c



Signed-off-by: Kai Meinhard <meinhard@gessler.de>