parisc: add missing forward declarations in asm/hardware.h

Fixes this warnings:

In file included from arch/parisc/include/asm/processor.h:15:0,
                 from arch/parisc/include/asm/spinlock.h:4,
                 from arch/parisc/include/asm/atomic.h:20,
                 from include/linux/atomic.h:4,
                 from arch/parisc/include/asm/bitops.h:11,
                 from include/linux/bitops.h:22,
                 from include/linux/kernel.h:19,
                 from include/linux/sched.h:55,
                 from arch/parisc/kernel/asm-offsets.c:31:
arch/parisc/include/asm/hardware.h:106:10: warning: ‘struct hardware_path’ declared inside parameter list [enabled by default]
arch/parisc/include/asm/hardware.h:106:10: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]
arch/parisc/include/asm/hardware.h:116:59: warning: ‘struct hardware_path’ declared inside parameter list [enabled by default]
arch/parisc/include/asm/hardware.h:118:47: warning: ‘struct hardware_path’ declared inside parameter list [enabled by default]
arch/parisc/include/asm/hardware.h:119:57: warning: ‘struct hardware_path’ declared inside parameter list [enabled by default]

Signed-off-by: Rolf Eike Beer <eike-kernel@sf-tec.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>