The special checks for whether we have a base IRQ offset or not
is surplus if we use the simple IRQ domain. The IRQ offset
zero will be interpreted as a linear domain case.

Plus this makes sure we allocate descriptors where need be, or
warn if they are preallocated with SPARSE_IRQ.

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>