Add board URLs for the RISC-V boards

Change-Id: Ifdf40986c2407d8c5b0097654b42e056f4498d39
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/14518
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/mainboard/emulation/qemu-riscv/board_info.txt

@@ -1,2 +1,3 @@
 Board name: QEMU RISCV
 Category: emulation
+Board URL: https://github.com/riscv/riscv-qemu

src/mainboard/emulation/spike-riscv/board_info.txt

@@ -1,2 +1,3 @@
 Board name: Spike RISCV
 Category: emulation
+Board URL: https://github.com/riscv/riscv-isa-sim