tests: spi: spi_loopback: add interrupt and DMA tests for GD32 boards

Add interrupt and DMA tests for GD32 boards.
Use overlay-gd32-spi-interrupt.conf when testing interrupt based transfer,
use overlay-gd32-spi-dma.conf when testing DMA transfer,

Add overlay file for GD32 boards for these tests.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>