arm: setup SPI IRQ routing tables

Since we soon start using GSI routing on ARM platforms too, we have
to setup the initial SPI routing table. Before the first call to
KVM_SET_GSI_ROUTING, the kernel holds this table internally, but this
is overwritten with the ioctl, so we have to explicitly set it up
here.
The routing is actually not used for IRQs triggered by KVM_IRQ_LINE,
but it needs to be here anyway. We use a simple 1:1 mapping.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>