gpio: Add support for microblaze xilinx GPIO

Microblaze uses gpio which is connected to the system reset.
Currently gpio subsystem wasn't used for it.

Add gpio driver and change Microblaze reset logic to be done
via gpio subsystem.

There are various configurations which Microblaze can have
that's why gpio_alloc/gpio_alloc_dual(for dual channel)
function has been introduced and gpio can be allocated
dynamically.

Adding several gpios IP is also possible and supported.

For listing gpio configuration please use "gpio status" command

This patch also remove one compilation warning:
microblaze-generic.c: In function 'do_reset':
microblaze-generic.c:38:47: warning: operation on '*1073741824u'
 may be undefined [-Wsequence-point]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>