qemuriscv64: Add the QEMU RISC-V 64-bit machine

qemuriscv64: Add the QEMU RISC-V 64-bit machine

The include is split ready to add the 32-bit RISC-V machine as soon as
glibc supports 32-bit RISC-V.

This is based on the work in the meta-riscv layer, thanks to Khem for
starting this.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>