sh: Wrap irq_to_desc_alloc_cpu() around CONFIG_SPARSE_IRQ temporarily.

irq_to_desc_alloc_cpu() has been renamed to irq_to_desc_alloc_node() in
-next, but as we can not presently enable SPARSE_IRQ without the early
irq_desc alloc patch, protect it with an ifdef until the interface has
settled and we are ready to enable it system-wide.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>