boards: riscv: tlsr9518adk80d: Enable GPIO and HEAP_MEM_POOL

- Enable GPIO by default to be consistent with other vendors,
Some samples also require GPIO driver to be enabled by default.
- Enable HEAP_MEM_POOL usage. The heap memory is used by Flash
driver during from flash to flash write operation.

Signed-off-by: Yuriy Vynnychek <yura.vynnychek@telink-semi.com>