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>
diff --git a/board/sifive/fu540/Kconfig b/board/sifive/fu540/Kconfig
index 86193d7..6f65681 100644
--- a/board/sifive/fu540/Kconfig
+++ b/board/sifive/fu540/Kconfig
@@ -65,5 +65,7 @@
 	imply SMP
 	imply MISC
 	imply SIFIVE_OTP
+	imply SYSRESET
+	imply SYSRESET_GPIO
 
 endif