net: macb: enable dcache in macb

net: macb: enable dcache in macb

Add to code to flush the dcache after we writing in DMA buffer.
Also we need invalidate the dcache before we check the status in the
DMA buffer.

Tested in SAMA5D3x-EK with gmac0. Tftp download speed shows in below:
	Disable DCache: 1.1 MiB/s
	Enable DCache: 1.6 MiB/s
Increase speed with about 40%.

The code should have no impact with the boards which are not
enable_dcache().
Tested in AT91SAM9M10G45EK.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>