of/irq: add empty irq_of_parse_and_map() for non-dt builds

Add a empty irq_of_parse_and_map() function that returns 0 for non-dt
builds and avoid having #ifdef CONFIG_OF around all calls to
irq_of_parse_and_map(). In addition to that, the irq_of_parse_and_map()
function declaration is made available only if CONFIG_OF_IRQ is defined,
which is the same config option that makes the irq_of_parse_and_map()
function definition available. While at it, fix a typo as well.

Changes since v1:
- Moved irq_of_parse_and_map() function declaration under CONFIG_OF_IRQ.
- Fix a minor typo in comments.

Suggested-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Rob Herring <rob.herring@calxeda.com>
[grant.likely: fix bug causing SPARC to break]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>