syscon: Compile the generic driver only when selected
syscon: Compile the generic driver only when selected

Currently the generic driver is always unconditionally compiled. Fix
this by compiling it only when needed.

Signed-off-by: Carlo Caione <ca4506c8c654a4ccf75802e86b5d162c1e96f065@baylibre.com>

finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr