boards: gd32e507v_start: add pyOCD support

The board can be programmed via the built-in GD-Link using pyOCD
(provided GigaDevice.GD32E50x_DFP CMSIS-Pack is installed. Update
instructions and set default runner to pyOCD since it does not require
any external tool like with J-Link.

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