Restructure sun4d_build_device_irq so that timer interrupts can be allocated

sun4d_build_device_irq was called without a valid platform_device when
the system timer was initialized on sun4d systems. This caused a NULL
pointer crash.

Josip Rodin suggested that the current sun4d_build_device_irq should be
split into two functions. So that the timer initialization could skip
the slot and sbus interface detection code in sun4d_build_device_irq, as
this does not make sence due to the timer interrupts not being generated
from a device located on sbus.

Signed-off-by: Kjetil Oftedal <oftedal@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>