UART3 is in the PER powerdomain.  If PER goes idle/inactive
independently of CORE, for UART3 to wakeup it must have its wakeup
enable bits setup in PM_WKEN_PER.  This patch enables these bits.

The reason it works when PER and CORE work together is because when
CORE goes inactive/retention, the IOPAD wakeups are enabled and
trigger UART3 wakeup.

Without this patch, when the UART inactivity timer fires for UART3,
its clocks are disabled and it's unable to wakeup so will be unusable
until PER is awoken by another source.

Another way of testing is by keeping CORE on during suspend but
allowing PER to hit retention

  # echo 3 > /debug/pm_debug/core_pwrdm/suspend

then enter suspend

  # echo mem > /sys/power/state

Without this patch, UART3 will be unable to wakeup the system.
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>