MIPS: Octeon: Make interrupt controller work with threaded handlers.

For CIUv1 controllers, we were relying on all calls to the irq_chip
functions to be done from the CPU that received the irq, and that they
would all be done from interrupt contest.  These assumptions do not
hold for threaded handlers.

We make all the masking actually mask the irq source, and use real
raw_spin_locks instead of manually twiddling the Status[IE] bit.

Signed-off-by: David Daney <david.daney@cavium.com>