riscv: sifive: fu540: Add gpio-restart support

The HiFive Unleashed board wires GPIO pin#10 to the input of the
system reset signal. This adds gpio reboot support.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Sagar Kadam <sagar.kadam@sifive.com>
Tested-by: Sagar Kadam <sagar.kadam@sifive.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>