socfpga: hotplug: put cpu1 in wfi

Use WFI when putting CPU1 to sleep.  Don't hold CPU1 in reset
since that results in increased power consumption.

Reset CPU1 briefly during CPU1 bootup.

This has been tested for hotplug and suspend/resume and results
in no increased power consumption.

Signed-off-by: Alan Tull <atull@opensource.altera.com>
Acked-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>