intc: swerv_pic: convert SYS_INIT to DEVICE_DT_INST_DEFINE

Convert SYS_INIT to DEVICE_DT_INST_DEFINE, this allows the build system
to track the device dependencies and ensure that the interrupt
controller is initialized before other devices using it.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>