boards: riscv: gd32vf103v_eval: initial support

Add initial support for the GD32VF103V-EVAL board. The board is based on
the GD32VF103 RISC-V MCU.

This board can run on Zephyr now largerly thanks to the initial work
done by @soburi.

Note that this board requires using the riscv-openocd fork, however,
programming is slow when using OpenOCD even though it works (including
debugging). J-Link option has also been enabled as it seems to be more
realiable and works _out of the box_. Some details are given in the
board documentation file.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>