Add a GPIO controller and use it to drive the shield I/O pins on the Arty

Add a GPIO controller and use it to drive the shield I/O pins on the Arty

This adds a GPIO controller which provides 32 bits of I/O.  The
registers are modelled on the set used by the gpio-ftgpio010.c driver
in the Linux kernel.  Currently there is no interrupt capability
implemented, though an interrupt line from the GPIO subsystem to the
XICS has been connected.

For the Arty A7 board, GPIO lines 0 to 13 are connected to the pins
labelled IO0 to IO13 on the "shield" connector, GPIO lines 14 to 29
connect to IO26 to IO41, GPIO line 30 connects to the pin labelled A
(aka IO42), and GPIO line 31 is connected to LED 7.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>