[ARM] 5101/2: EM-X270 updates

Convert EM-X270 pin configuration to use MFP tables and gpio library
Make device initialization dependent on respective driver CONFIG_ value (like
in zylonite)
Add keypad and gpio_key devices

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>