[ARM] pxa: add partial keypad support for mainstone

This is partial because mainstone's keypad is really special, some of
the keys like '1', '2', ... are actually connected to two row/column
juntions, thus pressing '1' is equivalent to pressing 'A' & 'H'.

This is really brain damanged since it makes distinguishing between
pressing '1' and multiple keys pressing of 'A' & 'H' difficult.

So these special keys are not supported for the time being.

Signed-off-by: eric miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>