mfd/cpuidle: ux500: Rename driver symbol

The PRCMU driver defines this as a DT node but there are no bindings
for it and it needs no data from the device tree. Just spawn the
device directly in the same way as the watchdog.

Name it "db8500-cpuidle" since there are no ambitions to support any
more SoCs than this one.

This rids this annoying boot message:
[    0.032610] cpuidle-dbx500: Failed to locate of_node [id: 0]

However I think the device still spawns and work just fine, despite
not finding a device tree node.

Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: linux-pm@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>