powerpc/mpic: Properly set default triggers

This gets rid of the unused default senses array, and replaces the
incorrect use of IRQ_TYPE_NONE with the new IRQ_TYPE_DEFAULT for
the initial set_trigger() call when mapping an interrupt.

This in turn makes us read the HW state and update the irq desc
accordingly.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>