intc: vexriscv_litex: 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.

Fix an incorrectly declared compatible and adjust some DT macros as
well.

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