QEMU model of the GPIO device on the SiFive E300 series SOCs.

The pins are not used by a board definition yet, however this
implementation can already be used to trigger GPIO interrupts from the
software by configuring a pin as both output and input.

Signed-off-by: Fabien Chouteau <chouteau@adacore.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>