gpio/langwell: allocate IRQ descriptors dynamically for SPARSE_IRQ

Since x86 is using SPARSE_IRQ by default nowadays it means that we need to
allocate IRQ descriptors dynamically using irq_alloc_descs() otherwise the
genirq code fails to convert our irq numbers to suitable descriptors.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>