[ARM] footbridge: add isa_init_irq() to common header

isa_init_irq() is defined in arch/arm/mach-footbridge/isa-irq.c
and used in arch/arm/mach-footbridge/common.c but there is no
definition in any header. Move the definition in common.c to
common.h to stop the sparse warning:

isa-irq.c:118:13: warning: symbol 'isa_init_irq' was not declared.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>