Input: pxa27x_keypad - rename the driver (was pxa27x_keyboard)

The controller should really be called keypad, and also align
the naming of functions and structures to use "pxa27x_keypad"
as prefix, instead of "pxakbd".

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>