ARM: dts: Add twl keypad map for LDP

Looks like we're still missing the keypad map for LDP.
Let's add it since we have the binding now available
for the twl4030_keypad as otherwise we get the following
errors during the boot:

twl4030_keypad keypad.31: OF: linux,keymap property not defined
in /ocp/i2c@48070000/twl@48/keypad
twl4030_keypad keypad.31: Failed to build keymap
twl4030_keypad: probe of keypad.31 failed with error -2

Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>