samples: flash_shell: add support for gd32 boards

After add zephyr,flash-controller property, most gd32 boards support
flash_shell sample.

gd32vf103c_starter and gd32vf103v_eval only have 32KB SRAM, so we
should reduce CONFIG_HEAP_MEM_POOL_SIZE to 8KB.

gd32f350r_eval only have 16KB driver, exclude from flash_shell sample.

Signed-off-by: HaiLong Yang <hailong.yang@brainco.cn>