i2c: Drop CONFIG_TSI108_I2C

Last user of this driver went away in June 2015 in commit
d928664f ("powerpc: 74xx_7xx: remove 74xx_7xx cpu support")

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
Acked-by: Heiko Schocher <hs@denx.de>