power/reset: vexpress: Fix restart/power off operation

The restart/power off implementation in the vexpress driver
used to obtain the config function when necessary. This was
wrong in two respects:

1. It required memory allocation with disabled interrupts
(it worked, but lockdep - when enabled - reported warnings).

2. Used jiffies-based timeout, while jiffies are not running
at this stage of system shutdown (therefore a config
transaction error - if happened - would have never be reported).

Fixed by pre-allocating the config function per device
and using mdelay for timeout.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>