ARM: dts: NSP: enable DMA on bcm988312hr

The previous patch "ARM: dts: NSP: Disable PL330 by default, add
dma-coherent property" set the DMAC to disabled by default, requiring it
to be manually enabled on each device. The bcm988312hr was mistakenly
omitted. This patch adds it back.

Signed-off-by: Matthew Hagan <mnhagan88@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>