boards: renesas: Maintain the minimal support of board RSK+ RZ/T2M

Renesas takes over the maintainer of board RSK+ RZ/T2M to unify with
other RZ boards

- Rename rzt2m_starter_kit to rzt2m_rsk
- Support xSPI boot mode to boot code from flash
- Change to use HAL Renesas

Signed-off-by: Nhut Nguyen <nhut.nguyen.kc@renesas.com>
Signed-off-by: Binh Nguyen <binh.nguyen.xw@renesas.com>