arch: hexagon: kernel: add export symbol function __delay()

Need add __delay() implementation, or can not pass allmodconfig in
next-20131118 tree.

The related error:

    CC      kernel/locking/spinlock_debug.o
  kernel/locking/spinlock_debug.c: In function '__spin_lock_debug':
  kernel/locking/spinlock_debug.c:114:3: error: implicit declaration of function '__delay' [-Werror=implicit-function-declaration]

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>