ARM: u8500_defconfig: turn on the Synaptics RMI4 driver

The Ux500 boards may have an RMI4 unit, and the DT fragments for
them have been merged upstream. Add it to the defconfig.

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