Blackfin: don't support keypad wakeup from hibernate

The on-chip keypad peripheral requires different registers to be setup
depending on the standby type (standby vs hibernation).  However, since
the power management framework doesn't differentiate between these types,
the driver doesn't know which registers to program and subsequently it
avoids doing so.

Always enabling the keyboard wakeup source causes misbehavior when the
pins are not assigned to the keypad.  If they happen to drive a certain
level, they'll trigger a wake up event which is not wanted.  So until
the aforementioned issue can be sorted out, drop support for the
wakeup source completely.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>