soc: arm: gigadevice: s/gigadevice/gd_gd32

Gigadevice was inconsistent with the convention established by other SoC
families, that is, use <vnd_prefix>_<family>. For example, ST STM32 uses
st_stm32. Note that GD32VF103, under soc/riscv, has already been
adjusted.

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