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

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

target/qemu-sifive-u/rules.mk

@@ -0,0 +1,23 @@
+LOCAL_DIR := $(GET_LOCAL_DIR)
+
+MODULE := $(LOCAL_DIR)
+
+PLATFORM := sifive
+VARIANT := sifive_u
+
+GLOBAL_DEFINES += SIFIVE_FREQ=5000000 # 5 MHz
+
+MEMBASE ?= 0x080000000
+MEMSIZE ?= 0x200000000 # 8 GiB
+
+WITH_SMP := 0
+SMP_MAX_CPUS := 4
+
+MODULE_SRCS := $(LOCAL_DIR)/target.c
+# set some global defines based on capability
+GLOBAL_DEFINES += PLATFORM_HAS_DYNAMIC_TIMER=1
+GLOBAL_DEFINES += ARCH_RISCV_CLINT_BASE=0x02000000
+GLOBAL_DEFINES += ARCH_RISCV_MTIME_RATE=10000000   # 10 MHz
+
+include make/module.mk
+