Unmask omap-keypad interrupt on suspend

Prevent keyboard interrupt from staying masked when omap-keypad driver
is suspended to allow this interrupt to trigger a system wake up.
Locks are used as suggested by Juha Yrjölä.

Signed-off-by: Andrzej Zaborowski <balrog@zabor.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>