mfd: Remove support for AB3100

The ST-Ericsson U300 platform has been removed, so this driver is no
longer needed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>