ARM: mach-shmobile: sh73a0 IRQ sparse alloc fix

Fix the sh73a0 external IRQ pin code to properly support
CONFIG_SPARSE_IRQ=y by allocating IRQ descriptors for the
cascaded IRQs associated with external IRQ pins.

Without this fix it is impossible to request IRQ0->IRQ31
on the Kota2 board when sparse IRQs are enabled.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>