[ARM] pxa/MioA701: change reset function to preserve RTC.

Change the halt and reboot method from gpio based to "jump
to ROM IPL beginning". This gives control back to IPL, which
without PowerOn key pressed, will put the device into deep
sleep until PowerOn is pressed for 1 second.

But this has the benefit of keeping the RTC registers across
reboots, which is good for OS change.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.miao@marvell.com>