1. use ioremap() for registers access, this improves the portability
   of the driver (e.g. same IP on different processor with different
   I/O memory range), and make it possible to remove those registers
   definition in pxa-regs.h as PXA is undergoing a clean-up of that
   header file

2. use device specific IRQ instead of hardcoded IRQ_KEYPAD, same
   reason as above

3. clean up the error handling path in _probe()

4. remove DRIVER_NAME and use pdev->name when necessary, we don't
   actually need a constant string literals
Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>