irqchip: Add irqchip_init dummy function

We add an empty irqchip_init dummy function for cases in which
CONFIG_IRQCHIP is not used. In these cases irqchip.c is not compiled,
but a funtion call may still be present in architecture code, that in
runtime doesn't get hit.

E.g. this is needed in the arch/arm/mach-shmobile/intc-r8a7740.c
interrupt setup code where OF use and non OF us is both handled in one
file.

Signed-off-by: Bastian Hecht <hechtb+renesas@gmail.com>
[horms+renesas@verge.net.au: Make non-CONFIG_IRQCHIP version static inline
 and remove trailing ';'.]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>