ARM: multi_v7_defconfig: enable the Meson PWM controller

This enables the Amlogic Meson PWM controller. It is used for a dimmable
LED on some devices. Additionally some devices use it to generate the
32.768 kHz (low power oscillator) clock.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>