MAINTAINERS: Add Arnd Bergmann as Syscon reviewer

The System Configuration (syscon) driver is an important one, which
effects many ARM-SoC based platforms.  I'm adding Arnd as a primary
reviewer as he is highly competent in this area and has a keen
interest in ARM-SoC.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>