[ARM] arch/arm/kernel/isa.c: missing definition of register_isa_ports

arch/arm/kernel/isa.c should include <linux/io.h> to get the
definition of register_io_ports() at-least when compiling for
footbridge to fix the following sparse warning:

isa.c:68:1: warning: symbol 'register_isa_ports' was not declared.

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