rpi3: Implement simple interrupt routing

Implement minimal interrupt routing functions. All interrupts are
treated as non-secure interrupts to be handled by the non-secure world.

Add note to the documentation about disabling FIQs qhen using OP-TEE
with Linux.

Change-Id: I937096542d973925e43ae946c5d0b306d0d95a94
Tested-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>