ARM: pxa: remove duplicated registeration on pxa-gpio

Both reboot (via reboot(RB_AUTOBOOT)) and suspend freeze on hx4700.

Registration of pxa_gpio_syscore_ops is moved into pxa-gpio driver,
but it still exists in arch-pxa directory. It resulsts failure on
reboot and suspend.

Now remove the registration code in arch-pxa.

Reported-by: Paul Parsons <lost.distance@yahoo.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>