hikey960: enable IOMCU DMAC

There exists a third DMA controller on the hi3660
SoC called the IOMCU DMAC. This controller is used by
peripherals like SPI2 and UART3. Initialize channels 4-7
as non-secure, while 0-3 remain reserved and secure.

Signed-off-by: Ryan Grachek <ryan@edited.us>