video: cfb_console: flush dcache for frame buffer in DRAM

Data cache flushing is required for frame buffer in RAM to fix the
distorted console text output. Currently this text distortion is
observed with cfb on beagleboard and N900 when running with data
cache enabled.

Reported-by: Pali Rohár <pali.rohar@gmail.com>
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>