same54: make the default irq handler weak
The default handler for all IRQs is the Dummy_Handler, which is a
simple while(true); wait-loop.
Introduce a weak symbol Default_Handler which alias to the
Dummy_Handler.
This allows replacing the default handler by a single
symbol rather than setting all IRQs seperate to the new handler.
Change-Id: I544e9c84dd538bb744fb799904adc58f9d850465
