of/irq: fix guards for irq_of_parse_and_map prototype

Since OF is now a userselectable config symbol, having OF=y but OF_IRQ=n
is a valid combination for non-OF platforms, and OF=y does not guarantee
anymore that OF_IRQ is enabled (or we are building for SPARC).

Fixes the following build error with OF=y, IRQ_DOMAIN=n and SPI=y:

drivers/built-in.o: In function `spi_register_master':
(.text+0xc3ae): undefined reference to `irq_of_parse_and_map'
Makefile:935: recipe for target 'vmlinux' failed
make: *** [vmlinux] Error 1

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Rob Herring <robh@kernel.org>