gpio-mpc8xxx: add mpc8xxx_gpio_set_multiple function

Add a set_multiple function to the MPC8xxx GPIO chip driver and thereby allow
for actual performance improvements when setting multiple outputs
simultaneously. In my case the time needed to configure an FPGA goes down from
48 s to 20 s.

Change log:
  v6: - rebase on current linux-gpio devel branch
  v5: - no change
  v4: - change interface of the set_multiple driver function to use
        unsigned long as type for the bit fields
      - use generic bitops (which also use unsigned long for bit fields)
  v3: - change commit message
  v2: - add this patch (v1 included only changes to gpiolib)

Signed-off-by: Rojhalat Ibrahim <imr@rtschenk.de>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>