net: stmmac: Add HW descriptor prefetch setting for DWMAC Core 5.20 onwards

DWMAC Core 5.20 onwards supports HW descriptor prefetching.
Additionally, it also depends on platform specific RTL configuration.
This capability could be enabled by setting DMA_Mode bit-19 (DCHE).

So, to enable this cability, platform must set plat->dma_cfg->dche = true
and the DWMAC core version must be 5.20 onwards. Else, this capability
wouldn`t be configured

Signed-off-by: Mohammad Athari Bin Ismail <mohammad.athari.ismail@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>