[ARM] 4185/2: entry: introduce get_irqnr_preamble and arch_ret_to_user

get_irqnr_preamble allows machines to take some action before entering the
get_irqnr_and_base loop.  On iop we enable cp6 access.

arch_ret_to_user is added to the userspace return path to allow individual
architectures to take actions, like disabling coprocessor access, before
the final return to userspace.

Per Nicolas Pitre's note, there is no need to cp_wait on the return to user
as the latency to return is sufficient.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>