of/irq: export of_get_irq_byname()

Similarly to of_get_irq(), let's export of_irq_get_byname(), so if a bus core
can be compiled as a module (such as I2C) it can have access to the symbol.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>