[platform][riscv-virt] added support for QEMU's riscv 'virt' machine

The virt machine is a generic target, much like the arm virt machine.
Intended to be simple to use and a good target to run large systems like
linux on. At the moment simply support booting and simple uart and timer
support.

target/qemu-virt-riscv/rules.mk

@@ -0,0 +1,7 @@
+LOCAL_DIR := $(GET_LOCAL_DIR)
+
+MODULE := $(LOCAL_DIR)
+
+PLATFORM := qemu-virt-riscv
+
+MEMSIZE ?= 0x01000000 # 16MB