ARM: picoxcell: don't reserve irq_descs

All irq_desc's are now dynamically allocated so we don't need to
statically reserve them.

v2:	- select SPARSE_IRQ and set .nr_irqs to NR_IRQS_LEGACY to skip
	ISA and IRQ 0.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>