Silicon/SynQuacer/PlatformDxe: set NETSEC DMA as coherent

We switched to cache coherent DMA for the NETSEC network controller ages
ago, but the platform driver that registers the non-discoverable device
currently does not reflect this change, which we haven't noticed since
the driver doesn't look at this flag.

Let's fix this nonetheless, in case it ever matters.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>