All other irq_domain_add_* functions are exported already, and apparently
this one got left out by mistake, which causes build errors for ARM
allmodconfig kernels:

ERROR: "irq_domain_add_simple" [drivers/gpio/gpio-rcar.ko] undefined!
ERROR: "irq_domain_add_simple" [drivers/gpio/gpio-em.ko] undefined!
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Acked-by: NSimon Horman <horms+renesas@verge.net.au>
Signed-off-by: NGrant Likely <grant.likely@linaro.org>