[target][riscv] Add support for qemu-sifive-u virt machine

A copy/paste from sifive-unleashed but without PWM/LED.

target/qemu-sifive-u/include/platform/sifive.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2019 Elliot Berman
+ *
+ * Use of this source code is governed by a MIT-style
+ * license that can be found in the LICENSE file or at
+ * https://opensource.org/licenses/MIT
+ */
+
+#pragma once
+
+#define SIFIVE_IRQ_UART0 4
+#define SIFIVE_IRQ_UART1 5
+
+#define SIFIVE_NUM_IRQS 127
+
+#define CLINT_BASE 0x02000000
+#define PLIC_BASE  0x0c000000
+#define UART0_BASE 0x10010000
+#define UART1_BASE 0x10011000
+
+#define PLIC_HART_IDX(hart)    (2 * (hart))